Ametek 7270 Instruction Manual

Model 7270

DSP Lock-in Amplifier

Instruction Manual

197852-A-MNL-E

Copyright © 2017 AMETEK ADVANCED MEASUREMENT TECHNOLOGY, INC

Firmware Version

The instructions in this manual apply to operation of a Model 7270 DSP Lock-in Amplifier that is fitted with
Version 2.20 or later operating firmware. Users of instruments that are fitted with earlier firmware
versions should update them to the current version free of charge by downloading an Update Pack from
our website at www.signalrecovery.com The pack includes full instructions for use.

Trademarks

AMETEK® and the b® and a logos are registered trademarks of AMETEK, Inc
Other product and company names mentioned are trademarks or trade names of their respective
companies.

Company Names

SIGNAL RECOVERY is part of Advanced Measurement Technology, Inc, a division of AMETEK, Inc. It
includes the businesses formerly trading as EG&G Princeton Applied Research, EG&G Instruments

(Signal Recovery), EG&G Signal Recovery and PerkinElmer Instruments (Signal Recovery).

Table of Contents

Table of Contents

i

General Safety Precautions .............................................................................................................. vii

Chapter One, Introduction

1.1 How to Use This Manual.................................................................................................................................. 1-1

1.2 What is a Lock-in Amplifier? ........................................................................................................................... 1-2

1.3 Key Specifications and Benefits ...................................................................................................................... 1-3

Chapter Two, Installation and Initial Checks

2.1 Installation ........................................................................................................................................................ 2-1

2.1.01 Introduction ............................................................................................................................................. 2-1

2.1.02 Rack Mounting ........................................................................................................................................ 2-1

2.1.03 Inspection ................................................................................................................................................ 2-1

2.1.04 Line Cord Plug ........................................................................................................................................ 2-1

2.1.05 Line Voltage Selection and Line Fuses ................................................................................................... 2-1

2.2 Initial Checks .................................................................................................................................................... 2-3

2.2.01 Introduction ............................................................................................................................................. 2-3

2.2.02 Procedure ................................................................................................................................................. 2-3

2.3 Line Frequency Filter Adjustment ................................................................................................................... 2-5

2.3.01 Introduction ............................................................................................................................................. 2-5

2.3.02 Procedure ................................................................................................................................................. 2-5

Chapter Three, Technical Description

3.1 Introduction ...................................................................................................................................................... 3-1

3.2 Operating Modes .............................................................................................................................................. 3-1

3.2.01 Introduction ............................................................................................................................................. 3-1

3.2.02 Single Reference / Dual Reference ......................................................................................................... 3-1

3.2.03 Tandem Demodulation ............................................................................................................................ 3-1

3.2.04 Single Harmonic / Dual Harmonic .......................................................................................................... 3-1

3.2.05 Internal / External Reference Mode ........................................................................................................ 3-2

3.2.06 Virtual Reference Mode .......................................................................................................................... 3-2

3.3 Principles of Operation ..................................................................................................................................... 3-2

3.3.01 Block Diagram ......................................................................................................................................... 3-2

3.3.02 Signal Channel Inputs.............................................................................................................................. 3-3

3.3.03 Line Frequency Rejection Filter .............................................................................................................. 3-4

3.3.04 AC Gain and Dynamic Reserve............................................................................................................... 3-4

3.3.05 Anti-Aliasing Filter ................................................................................................................................. 3-6

3.3.06 Main Analog-to-Digital Converter .......................................................................................................... 3-7

3.3.07 Reference Channel Inputs ....................................................................................................................... 3-7

3.3.08 Reference Channel .................................................................................................................................. 3-7

3.3.09 Phase-Shifter ........................................................................................................................................... 3-8

3.3.10 Internal Oscillator - General .................................................................................................................... 3-9

3.3.11 Internal Oscillator - Update Rate ............................................................................................................. 3-9

3.3.12 Internal Oscillator - Frequency & Amplitude Sweeps ............................................................................ 3-9

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ii

3.3.13 Internal Oscillator - Voltage Control .....................................................................................................3-10

3.3.14 Demodulators - Dual Phase Multipliers .................................................................................................3-10

3.3.15 Demodulators - Output Filters................................................................................................................3-10

3.3.16 Fast Curve Buffer ...................................................................................................................................3-11

3.3.17 Main Output Processor - General ...........................................................................................................3-11

3.3.18 Main Output Processor - Output Offset and Expand .............................................................................3-12

3.3.19 Main Output Processor - Vector Magnitude and Phase .........................................................................3-12

3.3.20 Main Output Processor - Noise Measurements ......................................................................................3-13

3.3.21 Main Output Processor - Standard Curve Buffer ...................................................................................3-14

3.3.22 Analog Outputs (DACs) .........................................................................................................................3-14

3.3.23 Auxiliary Analog Inputs (ADCs) ...........................................................................................................3-15

3.3.24 Main Microprocessor - General .............................................................................................................3-15

3.3.25 Main Microprocessor - Spectral Display ...............................................................................................3-15

3.3.26 Main Microprocessor - Auto Functions .................................................................................................3-15

3.3.27 Main Microprocessor - User Settings ....................................................................................................3-17

3.4 General ............................................................................................................................................................3-17

3.4.01 Accuracy .................................................................................................................................................3-17

3.4.02 Power-up Defaults ..................................................................................................................................3-17

Chapter Four, Front and Rear Panels

4.1 Front Panel ........................................................................................................................................................4-1

4.1.01 A and B (I) Signal Input Connectors ........................................................................................................4-1

4.1.02 REF IN Connector ....................................................................................................................................4-1

4.1.03 OSC OUT Connector ...............................................................................................................................4-1

4.1.04 LCD Screen ..............................................................................................................................................4-2

4.1.05 HELP Key ................................................................................................................................................4-5

4.1.06 MENU Key ..............................................................................................................................................4-5

4.1.07 SELECT CONTROL Key ........................................................................................................................4-5

4.2 Rear Panel .........................................................................................................................................................4-6

4.2.01 Line Power Switch ...................................................................................................................................4-6

4.2.02 Line Power Input Assembly .....................................................................................................................4-6

4.2.03 DIGITAL I/O Connector ..........................................................................................................................4-6

4.2.04 USB Connector ........................................................................................................................................4-6

4.2.05 LAN Connector ........................................................................................................................................4-7

4.2.06 RS232 Connector .....................................................................................................................................4-7

4.2.07 AUX RS232 Connector ............................................................................................................................4-7

4.2.08 PRE-AMP POWER Connector ................................................................................................................4-7

4.2.09 ADC TRIG IN Connector ........................................................................................................................4-7

4.2.10 ADC 1, ADC 2, ADC 3 and ADC 4 Connectors .....................................................................................4-7

4.2.11 REF MON Connector ...............................................................................................................................4-7

4.2.12 TTL REF IN Connector ...........................................................................................................................4-7

4.2.13 TRIG IN Connector ..................................................................................................................................4-8

4.2.14 TRIG OUT Connector ..............................................................................................................................4-8

4.2.15 DAC 1, DAC 2, DAC 3, and DAC 4 Connectors ....................................................................................4-8

4.2.16 SIG MON Connector................................................................................................................................5-8

Chapter Five, Front Panel Operation

5.1 Introduction .......................................................................................................................................................5-1

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iii

5.2 Menu Structure ................................................................................................................................................. 5-2

5.3 Menu Descriptions - Single Reference Mode .................................................................................................. 5-3

5.3.01 Main Display ........................................................................................................................................... 5-3

5.3.02 Control Selection Menu .......................................................................................................................... 5-5

5.3.03 Main Menu 1 ........................................................................................................................................... 5-7

5.3.04 Signal Channel Menu .............................................................................................................................. 5-7

5.3.05 Reference Channel Menu ...................................................................................................................... 5-11

5.3.06 Output Filters Menu .............................................................................................................................. 5-13

5.3.07 Output Offset & Expand Menu ............................................................................................................. 5-14

5.3.08 Output Equations Menu......................................................................................................................... 5-15

5.3.09 Oscillator Menu ..................................................................................................................................... 5-17

5.3.10 Frequency Sweep Menu ........................................................................................................................ 5-18

5.3.11 Amplitude Sweep Menu ........................................................................................................................ 5-21

5.3.12 Amplitude Modulation Menu ................................................................................................................ 5-23

5.3.13 Frequency Modulation (VCO) Menu .................................................................................................... 5-24

5.3.14 Auto Functions Menu ............................................................................................................................ 5-25

5.3.15 Configuration Menu 1 ........................................................................................................................... 5-28

5.3.16 Communications Menu ......................................................................................................................... 5-30

5.3.17 RS232 Settings Menu ............................................................................................................................ 5-30

5.3.18 Ethernet Settings Menu ......................................................................................................................... 5-32

5.3.19 USB Status Menu .................................................................................................................................. 5-34

5.3.20 Communications Monitor ...................................................................................................................... 5-35

5.3.21 Options Menu ........................................................................................................................................ 5-36

5.3.22 Configuration Menu 2 ........................................................................................................................... 5-36

5.3.23 Spectral Display .................................................................................................................................... 5-37

5.3.24 Main Menu 2 ......................................................................................................................................... 5-39

5.3.25 Curve Buffer Menu ............................................................................................................................... 5-40

5.3.26 Curve Trigger Menu .............................................................................................................................. 5-42

5.3.27 Curve Select Menu ................................................................................................................................ 5-44

5.3.28 Single Graph Menu ............................................................................................................................... 5-45

5.3.29 Double Graph Menu .............................................................................................................................. 5-46

5.3.30 User Settings Menu ............................................................................................................................... 5-48

5.3.31 ADC Menu ............................................................................................................................................ 5-49

5.3.32 DAC Menu ............................................................................................................................................ 5-50

5.3.33 Digital Port Menu .................................................................................................................................. 5-54

5.4 Menu Descriptions - Virtual Reference Mode ............................................................................................... 5-55

5.4.01 Virtual Reference Menus ...................................................................................................................... 5-55

5.4.02 Virtual Reference Main Display ........................................................................................................... 5-58

5.4.03 Virtual Reference Configuration Menu................................................................................................. 5-59

5.5 Menu Descriptions - Dual Reference Mode ................................................................................................... 5-59

5.5.01 Dual Reference Setup Menu.................................................................................................................. 5-59

5.5.02 Dual Reference Main Display ............................................................................................................... 5-60

5.5.03 Dual Reference Channel Menu 1 .......................................................................................................... 5-64

5.5.04 Dual Reference Channel Menu 2 .......................................................................................................... 5-66

5.5.05 Dual Reference Output Filters Menu 1 ................................................................................................. 5-66

5.5.06 Dual Reference Output Filters Menu 2 ................................................................................................. 5-68

5.5.07 Dual Reference Output Offset Ref 1 Menu ........................................................................................... 5-69

5.5.08 Dual Reference Output Offset Ref 2 Menu ........................................................................................... 5-70

5.5.09 Dual Reference Auto Functions Menus ................................................................................................ 5-71

5.5.10 Dual Reference Configuration Menu .................................................................................................... 5-72

5.5.11 Dual Reference and Dual Harmonic Modes Curve Select Menu .......................................................... 5-72

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iv

5.5.12 Dual Reference and Dual Harmonic Modes DAC Menu .......................................................................5-73

5.6 Menu Descriptions - Dual Harmonic Mode ....................................................................................................5-78

5.6.01 Dual Harmonic Setup Menu ...................................................................................................................5-78

5.6.02 Dual Harmonic Main Display ................................................................................................................5-78

5.6.03 Dual Harmonic Reference Channel Menu .............................................................................................5-82

5.6.04 Dual Harmonic Output Filters Menu 1 ..................................................................................................5-84

5.6.05 Dual Harmonic Output Filters Menu 2 ..................................................................................................5-85

5.6.06 Dual Harmonic Output Offset Harm 1 Menu .........................................................................................5-86

5.6.07 Dual Harmonic Output Offset Harm 2 Menu .........................................................................................5-87

5.6.08 Dual Harmonic Auto Functions Menus .................................................................................................5-88

5.6.09 Dual Harmonic Configuration Menu .....................................................................................................5-89

5.7 Typical Lock-in Amplifier Experiment ...........................................................................................................5-89

Chapter Six, Computer Operation

6.1 Introduction .......................................................................................................................................................6-1

6.2 Capabilities ........................................................................................................................................................6-1

6.2.01 General .....................................................................................................................................................6-1

6.2.02 Operation ..................................................................................................................................................6-1

6.2.03 Communications Monitor Menu ..............................................................................................................6-1

6.2.04 Compound Commands .............................................................................................................................6-1

6.3 RS232 Operation ...............................................................................................................................................6-1

6.3.01 Introduction ..............................................................................................................................................6-1

6.3.02 General Features .......................................................................................................................................6-2

6.3.03 Choice of Baud Rate ................................................................................................................................6-2

6.3.04 Choice of Number of Data Bits ................................................................................................................6-3

6.3.05 Choice of Parity Check Option ................................................................................................................6-3

6.3.06 Auxiliary RS232 Interface .......................................................................................................................6-3

6.3.07 Handshaking and Echoes .........................................................................................................................6-3

6.3.08 Terminators ..............................................................................................................................................6-4

6.3.09 Delimiters .................................................................................................................................................6-4

6.3.10 Status Byte, Prompts and Overload Byte .................................................................................................6-4

6.4 USB Operation ..................................................................................................................................................6-5

6.4.01 Introduction ..............................................................................................................................................6-5

6.4.02 General Features .......................................................................................................................................6-6

6.4.03 Terminator, Status Byte, and Overload Byte ...........................................................................................6-6

6.4.04 Delimiters .................................................................................................................................................6-6

6.5 Ethernet Operation ............................................................................................................................................6-7

6.5.01 Introduction ..............................................................................................................................................6-7

6.5.02 IP Address ................................................................................................................................................6-7

6.5.04 Home Page ...............................................................................................................................................6-7

6.5.03 Sockets .....................................................................................................................................................6-8

6.5.04 Terminator, Status Byte, and Overload Byte ...........................................................................................6-8

6.5.05 Delimiters .................................................................................................................................................6-9

6.6 Command Format ..............................................................................................................................................6-9

6.7 Command Descriptions ...................................................................................................................................6-10

6.7.01 Signal Channel .......................................................................................................................................6-10

6.7.02 Reference Channel .................................................................................................................................6-13

6.7.03 Signal Channel Output Filters ................................................................................................................6-15

6.7.04 Signal Channel Output Amplifiers .........................................................................................................6-17

6.7.05 Instrument Outputs .................................................................................................................................6-18

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v

6.7.06 Internal Oscillator .................................................................................................................................. 6-20

6.7.07 Analog Outputs ...................................................................................................................................... 6-24

6.7.08 Digital I/O .............................................................................................................................................. 6-25

6.7.09 Auxiliary Inputs ..................................................................................................................................... 6-26

6.7.10 Output Data Curve Buffer ..................................................................................................................... 6-26

6.7.11 Computer Interfaces .............................................................................................................................. 6-33

6.7.12 Instrument Identification ....................................................................................................................... 6-35

6.7.13 Front Panel ............................................................................................................................................ 6-35

6.7.14 Auto Default and Calibration ................................................................................................................ 6-36

6.7.15 Dual Mode Commands .......................................................................................................................... 6-36

6.5 Programming Examples ................................................................................................................................. 6-37

6.5.01 Introduction ........................................................................................................................................... 6-37

6.5.02 Basic Signal Recovery ........................................................................................................................... 6-37

6.5.03 Frequency Response Measurement ....................................................................................................... 6-38

6.5.04 X and Y Output Curve Storage Measurement ....................................................................................... 6-38

Appendix A, Specifications

Appendix B, Pinouts

B1 RS232 Connector Pinout ................................................................................................................................. B-1

B2 Preamplifier Power Connector Pinout .............................................................................................................. B-1

B3 Digital Output Port Connector .......................................................................................................................... B-2

Appendix C, Cable Diagrams

C1 RS232 Cable Diagrams ...................................................................................................................................... C1

Appendix D, Default Settings

Auto Default Function ............................................................................................................................................. D1

Appendix E, Alphabetical Listing of Commands

Index

Warranty ...................................................................................................................................... End of Manual

TABLE OF CONTENTS

vi

GENERAL SAFETY PRECAUTIONS

The equipment described in this manual has been designed in accordance with EN61010 "Safety
requirements for electrical equipment for measurement, control and laboratory use", and has been
supplied in a safe condition. To avoid injury to an operator or service technician the safety precautions
given below, and throughout the manual, must be strictly adhered to, whenever the equipment is
operated, serviced or repaired. For specific safety details, please refer to the relevant sections within
the manual.

The equipment is intended solely for electronic measurement and should be used for no other purpose.
SIGNAL RECOVERY accepts no responsibility for accidents or damage resulting from any failure to

comply with these precautions.

Grounding

To minimize the hazard of electrical shock, it is essential that the equipment be connected to a
protective ground through the AC supply cable. The continuity of the ground connection should be
checked periodically.

AC Supply Voltage

Never operate the equipment from a line voltage or frequency in excess of that specified. Otherwise,
the insulation of internal components may break down and cause excessive leakage currents.

Fuses

Before switching on the equipment check that the fuses accessible from the exterior of the equipment
are of the correct rating. The rating of the AC line fuse must be in accordance with the voltage of the
AC supply.

Should any fuse continually blow, do not insert a fuse of a higher rating. Switch the equipment off,
clearly label it "unserviceable" and inform a service technician.

Explosive Atmospheres

This equipment must NEVER BE OPERATED in a potentially explosive atmosphere. The equipment is
NOT designed for use in these conditions and could possibly cause an explosion.

Safety Symbols

For the guidance and protection of the user, the following safety symbols may appear on the
equipment, together with details of the hazard where appropriate:

Symbol

Meaning

General safety hazard. Refer to the operating manual for detailed instructions.

Electrical safety hazard. This symbol may appear alongside the general safety
hazard symbol, together with a voltage.

Notes and Cautions

For the guidance and protection of the user, Notes and Cautions appear throughout the manual. The
significance of these is as follows:

NOTES highlight important information for the reader’s special attention.
CAUTIONS guide the reader in avoiding damage to the equipment.

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vii

Avoid Unsafe Equipment

The equipment may be unsafe if any of the following statements apply:

 Equipment shows visible damage.
 Equipment has failed to perform an intended operation.
 Equipment has been stored in unfavorable conditions.
 Equipment has been subjected to severe physical stress.

If in any doubt as to the serviceability of the equipment, don't use it. Get it properly checked out by a
qualified service technician.

Live Conductors

When the equipment is connected to its measurement inputs or supply, the opening of covers or
removal of parts could expose live conductors. The equipment must be disconnected from all power
and signal sources before it is opened for any adjustment, replacement, maintenance or repair.
Adjustments, maintenance or repair must only be done by qualified personnel, who should refer to the
relevant maintenance documentation.

Equipment Modification

To avoid introducing safety hazards, never install non-standard parts in the equipment, or make any
unauthorized modification. To maintain safety, always return the equipment to your
SIGNAL RECOVERY service provider for service and repair.

European WEEE Directive

This product is subject to Directive 2002/96/EC of the European Parliament and the Council of the
European Union on waste electrical and electronic equipment (WEEE) and, in jurisdictions adopting
that Directive, is marked as being put on the market after August 13, 2005, and should not be disposed
of as unsorted municipal waste. Please use your local WEEE collection facilities for the disposal of this
product and otherwise observe all applicable requirements.

FCC Notice

This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used
in accordance with this instruction manual, may cause harmful interference with radio communications.
Operation of this equipment in a residential area is likely to cause harmful interference, in which case
the user is required to correct the interference at his own expense.

Acknowledgment

Operation of the Ethernet interface in the model 7270 relies on software code developed by the
Swedish Institute of Computer Science, copyright 2001-2004, all rights reserved. In accordance with
the license under which it is used, we reproduce here the following disclaimer:

THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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viii

DECLARATION OF CONFORMITY

The directives covered by this declaration

73/23/EEC Low Voltage Equipment Directive, amended by 93/68/EEC
89/336/EEC Electromagnetic Compatibility Directive, amended by 92/31/EEC

& 93/68/EEC

Product(s)

Model 7270 DSP Lock-in Amplifier

Basis on which conformity is being declared

The product(s) identified above comply with the requirements of the EU directives by
meeting the following standards:

BS EN61326:1998 Electrical equipment for measurement control and laboratory use -

EMC requirements; including amendments A1:1998 and A2:2001.

BS EN61010-1:2001 Safety requirements for electrical equipment for measurement,

control and laboratory use.

Accordingly the CE mark has been applied to this product.

Signed
For and on behalf of SIGNAL RECOVERY

Authority: Business Element Manager
Date: May 2009

Introduction

Chapter 1

1-1

1.1 How to Use This Manual

This manual gives detailed instructions for setting up and operating the
SIGNAL RECOVERY Model 7270 DSP Lock-in Amplifier. It is split into the

following chapters:-

Chapter 1 - Introduction

Provides an introduction to the manual, briefly describes the function of a lock-in
amplifier and the types of measurements it may be used for, and lists the major
specifications of the model 7270.

Chapter 2 - Installation and Initial Checks

Describes how to install the instrument and gives a simple test procedure which may
be used to check that the unit has arrived in full working order.

Chapter 3 - Technical Description

Provides an outline description of the design of the instrument and discusses the
effect of the various controls. A good understanding of the design will enable the
user to get the best possible performance from the unit.

Chapter 4 - Front and Rear Panels

Describes the instrument’s connectors, controls and indicators as referred to in the

subsequent chapters.

Chapter 5 - Front Panel Operation

Describes the capabilities of the instrument when used as a manually operated unit,
and shows how to operate it using the front panel controls.

Chapter 6 - Computer Operation

This chapter provides detailed information on operating the instrument from a
computer via the built-in interfaces. It includes information on how to establish
communications, the functions available, the command syntax and a detailed
command listing.

Appendix A

Gives the detailed specifications of the unit.

Appendix B

Details the pinouts of the multi-way connectors on the rear panel of the unit.

Appendix C

Shows the connection diagrams for suitable RS232 null-modem cables to couple the
unit to a compatible computer.

Appendix D

Provides a listing of the instrument settings produced by using the Auto-Default
functions.

Appendix E

Gives an alphabetical listing of the computer commands for easy reference.
New users are recommended to unpack the instrument and carry out the procedure in

chapter 2 to check that it is working satisfactorily. They should then make
themselves familiar with the information in chapters 3, 4 and 5, even if they intend
that the unit will eventually be used under computer control. Only when they are

Chapter 1, INTRODUCTION

1-2

fully conversant with operation from the front panel should they then turn to chapter
6 for information on how to use the instrument remotely. Once the structure of the
computer commands is familiar, appendix E will prove convenient as it provides a
complete alphabetical listing of these commands in a single easy-to-use section.

1.2 What is a Lock-in Amplifier?

Since their invention back in the 1960's, lock-in amplifiers have been used whenever
the need arises to measure the amplitude and/or phase of a signal of known
frequency in the presence of noise. Unlike other AC measuring instruments they
have the ability to give accurate results even when the noise is much larger than the
signal - in favorable conditions even up to a million times larger.

Early instruments used analog technology, with manual controls and switches, and
with output readings being taken from large panel meters. Later, microprocessors
were added to give more user-friendly operation, digital output displays, and to
support computer control. In current designs the analog phase sensitive detectors
forming the heart of the instrument have been replaced by DSP (digital signal
processing) designs, further improving performance.

The model 7270 DSP lock-in amplifier uses the latest DSP technology for signal
detection, and a powerful processor for easy user operation. The low-noise analog
signal channel, with its choice of input mode and impedance, complements the
digital technology, giving an instrument that will be of use in many fields of
scientific research, such as optics, electrochemistry, materials science, fundamental
physics and electrical engineering.

In these and other experiments it can function as a:­  AC Signal Recovery Instrument  Transient Recorder
 Vector Voltmeter  DSP Oscillator
 Phase Meter  Frequency Meter
 Spectrum Analyzer  Noise Measurement Unit
These characteristics, all available in a single compact console, make it an invaluable

addition to any laboratory.

Chapter 1, INTRODUCTION

1-3

1.3 Key Specifications and Benefits

The SIGNAL RECOVERY Model 7270 represents a further significant advance in
the application of DSP technology in the design of a lock-in amplifier.

Key specifications include:

 Frequency range: 0.001 Hz to 250.000 kHz
 Voltage sensitivity: 2 nV to 1 V full-scale
 Current input mode sensitivities: 2 fA to 1 µA full-scale

2 fA to 10 nA full-scale

 Line frequency rejection filter
 Dual phase demodulator with X-Y and R- outputs
 Very low phase noise of < 0.0001° rms
 Output time constant: 10 µs to 100 ks
 5-digit output readings
 Dual reference mode - allows simultaneous measurement of two signals at

different reference frequencies.

 Single and dual harmonic mode - allows simultaneous measurement of up to two

different harmonics of a signal

 Tandem demodulation capability - suitable for double demodulation experiments

that would previously have required two lock-in amplifiers

 Spectral Display mode shows frequency spectrum of the signal prior to the

demodulators to help in selecting a reference frequency

 Virtual reference mode - allows reference free measurements
 Direct Digital Synthesizer (DDS) oscillator with variable amplitude and

frequency

 Oscillator frequency and amplitude sweep generator
 Voltage controlled oscillator frequency or amplitude
 8-bit programmable digital I/O port for external system control
 Four configurable DAC outputs which can be used as analog signal outputs

and/or as auxiliary DAC outputs

 Four auxiliary ADC inputs
 Full range of auto functions
 Non-volatile memory for 8 complete instrument settings
 Standard USB, Ethernet, and RS232 interfaces with RS232 daisy-chain

capability for up to 16 instruments.

 Large high-resolution color LCD display panel with menus for control and

display of instrument outputs in both digital and graphical formats

 Easy entry of numerical control settings using keypad
 100,000 point internal curve storage buffer

Chapter 1, INTRODUCTION

1-4

Installation and
Initial Checks

Chapter 2

2-1

2.1 Installation

2.1.01 Introduction

Installation of the model 7270 is very straightforward. The instrument can be
operated on almost any laboratory bench or be rack mounted at the user's

convenience. With an ambient operating temperature range of 0 C to 35 C, it is
highly tolerant to environmental variables, needing only to be protected from
exposure to corrosive agents and liquids.

The model 7270 uses forced-air ventilation and as such should be located so that the
ventilation holes on the side and rear panels are not obstructed. This condition is best
satisfied by leaving a space of at least 2" (5 cm) between these panels and any
adjacent surface.

2.1.02 Rack Mounting

An optional accessory kit, part number K02005, is available from
SIGNAL RECOVERY to allow the model 7270 to be mounted in a standard 19-inch

rack.

2.1.03 Inspection

Upon receipt the model 7270 Lock-in Amplifier should be inspected for shipping
damage. If any is noted, SIGNAL RECOVERY should be notified immediately and

a claim filed with the carrier. The shipping container should be saved for inspection
by the carrier.

2.1.04 Line Cord Plug

The model 7270 is fitted with a standard IEC 320 input socket on its rear panel and a
suitable line cord is supplied.

2.1.05 Line Voltage Selection and Line Fuses

Before plugging in the line cord, ensure that the model 7270 is set to the voltage of
the AC power supply to be used.

A detailed discussion of how to check and, if necessary, change the line voltage
setting follows.

CAUTION: The model 7270 may be damaged if the line voltage is set for 110 V
AC operation and is turned on with 220 V AC applied to the power input
connector.

The instrument can operate from any one of four different line voltage ranges, 90­110 V, 110-130 V, 200-240 V, and 220-260 V, at 50-60 Hz. The change from one
range to another is made by repositioning the plug-in barrel selector internal to the
line input assembly on the rear panel of the unit. Instruments are normally shipped
from the factory with the line voltage selectors set to 110-130 V AC, unless they are
destined for an area known to use a line voltage in the 220-260 V range, in which
case, they are shipped configured for operation from the higher range.

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-2

The line voltage setting can be seen through a small rectangular window in the line
input assembly on the rear panel of the instrument (figure 2-1). If the number
showing is incorrect for the local line voltage (refer to table 2-1), then the barrel
selector will need to be repositioned as follows.

Observing the instrument from the rear, note the plastic door forming part of the
input assembly (figure 2-1). When the line cord is removed from the rear-panel
connector, the plastic door can be opened outwards by placing a small, flat-bladed
screwdriver in the slot and levering gently. This gives access to the fuse and to the
voltage barrel selector. Remove the barrel selector with the aid of a small
screwdriver or similar tool. With the barrel selector removed, four numbers become
visible on it: 100, 120, 220, and 240, only one of which is visible when the door is
closed. Table 2-1 indicates the actual line voltage range represented by each number.
Position the barrel selector such that the required number (see table 2-1) will be
visible when the barrel selector is inserted and the door closed.

Figure 2-1, Line Input Assembly

VISIBLE # VOLTAGE RANGE
100 90 - 110 V
120 110 - 130 V
230 200 - 240 V
240 220 - 260 V

Table 2-1, Range vs. Barrel Position

Next check the fuse rating. For operation from a nominal line voltage of 100 V or
120 V, use a 20 mm slow-blow fuse rated at 1.0 A, 250 V. For operation from a
nominal line voltage of 220 V or 240 V, use a 20 mm slow-blow fuse rated at 0.5 A,
250 V.

To change the fuse, first remove the fuse holder by pulling the plastic tab marked
with an arrow. Remove the fuse and replace with a slow-blow fuse of the correct
voltage and current rating. Install the fuse holder by sliding it into place, making sure
the arrow on the plastic tab is pointing downwards. When the proper fuse has been
installed, close the plastic door firmly. The correct selected voltage setting should
now be showing through the rectangular window. Ensure that only fuses with the
required current and voltage ratings and of the specified type are used for
replacement. The use of makeshift fuses and the short-circuiting of fuse holders is
prohibited and potentially dangerous.

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-3

2.2 Initial Checks

2.2.01 Introduction

The following procedure checks the performance of the model 7270. In general, this
procedure should be carried out after inspecting the instrument for obvious shipping
damage.

NOTE: Any damage must be reported to the carrier and to SIGNAL RECOVERY
immediately. In addition the shipping container must be retained for inspection by

the carrier.

Note that this procedure is intended to demonstrate that the instrument has arrived in
good working order, not that it meets specifications. Each instrument receives a
careful and thorough checkout before leaving the factory, and normally, if no
shipping damage has occurred, will perform within the limits of the quoted
specifications. If any problems are encountered in carrying out these checks, contact
SIGNAL RECOVERY or the nearest authorized representative for assistance.

2.2.02 Procedure

1) Ensure that the model 7270 is set to the line voltage of the power source to be
used, as described in section 2.1.05.

2) With the rear-panel mounted power switch set to 0 (off), plug in the line cord to
an appropriate line power source.

3) Turn the model 7270 power switch to the I (on) position.

4) The front panel display will now briefly display the following:-

Figure 2-2, Opening Display

5) Wait until the opening display has changed to the Main Display and then press
the key under the bottom right hand corner of the display identified by the legend
MENU on the display. This enters the first of the two main menus,
Main Menu 1, shown below in figure 2-3.

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-4

Figure 2-3, Main Menu 1

6) Press one of the keys adjacent to the Auto functions menu item to enter the Auto
Functions menu, shown below in figure 2-4.

Figure 2-4, Auto Functions Menu

7) Press one of the keys adjacent to the Auto Default menu item. This will set all of
the instrument's controls and the display to a defined state. The display will
revert to the Main Display, as shown below in figure 2-5, with the right-hand
side showing the vector magnitude, R, and the phase angle, , of the measured
signal in digital form, with two bar-graphs showing the X channel output and Y
channel output expressed in millivolts. The left-hand side shows five instrument
controls, these being the AC Gain in decibels, full-scale sensitivity, output time
constant, reference phase and internal oscillator frequency. The resulting
dynamic reserve (DR), in decibels, is also shown.

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-5

Figure 2-5, Main Display

8) Connect a BNC cable between the OSC OUT and A input connectors on the

front panel of the instrument.

9) The right-hand side of the display should now indicate R, the vector magnitude,
close to 100% of full-scale (i.e. the sinusoidal oscillator output, which was set to
1 kHz and a signal level of 0.2 V rms by the Auto-Default function, is being
measured with a full-scale sensitivity of 200 mV rms) and , the phase angle, of
near zero degrees, if a short cable is used.

This completes the initial checks. Even though the procedure leaves many functions
untested, if the indicated results were obtained then the user can be reasonably sure
that the unit incurred no hidden damage in shipment and is in good working order.

2.3 Line Frequency Filter Adjustment

2.3.01 Introduction

The model 7270 incorporates a line-frequency rejection filter, which is normally
supplied set to 60 Hz. If the power line frequency of the country in which the
instrument is to be used is also 60 Hz then the setting does not need to be changed.
If, however, the unit is to be used in an area with a 50 Hz power line frequency the
setting should be changed using the following procedure.

2.3.02 Procedure

1) Turn the model 7270 power switch to the I (on) position.

2) The instrument's front panel display will now briefly display the following:-

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-6

Figure 2-6, Opening Display

4) Wait until the opening display has changed to the Main Display and then press
the key under the bottom right hand corner of the display identified by the legend
MENU on the display once. This enters the first of the two main menus,
Main Menu 1, shown below in figure 2-7.

Figure 2-7, Main Menu 1

4) Press one of the keys adjacent to the Configuration menu item to enter the
Configuration menu 1, shown below in figure 2-8.

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-7

Figure 2-8, Configuration Menu 1

5) Press one of the keys adjacent to the Configuration 2 menu item to enter the
Configuration menu 2, shown below in figure 2-9.

Figure 2-9, Configuration Menu 2

5) The present line frequency setting is shown under the LINE FREQUENCY label
and is either 50 or 60 Hz. In figure 2-9, the filter is set to 60 Hz. If this setting
does not match the local line frequency, then press a key adjacent to this item
once to change it.

6) Press the key marked MAIN DISPLAY once to return to the Main Display.

This completes the procedure for adjusting the line filter frequency.

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-8

Technical Description

Chapter 3

3-1

3.1 Introduction

The model 7270 lock-in amplifier is a sophisticated instrument with many
capabilities beyond those found in other lock-in amplifiers. This chapter discusses
the various operating modes provided and then describes the design of the instrument
by considering it as a series of functional blocks. In addition to describing how each
block operates, the sections also include information on the effect of the various
controls.

3.2 Operating Modes

3.2.01 Introduction

The model 7270 incorporates a number of different operating modes which are
referred to in the following technical description, so in order to help the reader's
understanding they are defined here.

3.2.02 Single Reference / Dual Reference

Conventionally, a lock-in amplifier makes measurements such as signal magnitude,
phase, etc. on the applied signal at a single reference frequency. In the model 7270
this is referred to as the single reference mode.

The dual reference mode incorporated in the model 7270 allows the instrument to
make simultaneous measurements at two different reference frequencies, an ability
that previously required two lock-in amplifiers. This flexibility incurs a few
restrictions, most notably that both signals be passed through the same input signal
channel, which implies either that both signals are derived from the same detector
(for example two chopped light beams falling onto a single photodiode) or that they
can be summed prior to measurement, either externally or by using the differential
input mode of the instrument. Nevertheless, the mode will prove invaluable in many
experiments. Note that the restriction that that one reference frequency be from the
internal oscillator and one from an external source which used to apply is removed
for instruments with the latest firmware, allowing dual reference mode operation
with two external reference signals. However, in this case one of the references is
limited to a maximum of 3 kHz.

3.2.03 Tandem Demodulation

A further development of the dual reference mode is Tandem Demodulation. In this
mode, the input to the second set of demodulators is taken not from the main ADC as
is the case with normal dual reference mode, but from the filtered X-channel output
of the first set of demodulators. Hence, for example, the mode can be used to
measure the modulation amplitude of an amplitude-modulated “carrier” frequency.
The carrier frequency is at the internal reference frequency, and so this can be
detected by the first stage demodulators. If the output time constant of this first stage
is short enough, then the X output will represent a signal at the modulation
frequency. The second set of demodulators, this time operating at the modulation
frequency, then measure the amplitude and/or phase of this modulation.

3.2. Single Harmonic / Dual Harmonic

Normally, a lock-in amplifier measures the applied signal at the reference frequency.

Chapter 3, TECHNICAL DESCRIPTION

3-2

However, in some applications such as Auger Spectroscopy and amplifier
characterization, it is useful to be able to make measurements at some multiple n, or
harmonic, of the reference frequency, f. The model 7270 allows this multiple to be
set to any value between 2 (i.e. the second harmonic) and 127, as well as unity,
which is the normal mode. The only restriction is that the product n × f cannot
exceed 250 kHz.

Dual harmonic mode allows the simultaneous measurement of two different
harmonics of the input signal, subject only to the restriction that the maximum value
of n × f is 250 kHz.

3.2.05 Internal / External Reference Mode

In the internal reference mode, the instrument's reference frequency is derived from
its internal oscillator and the oscillator signal is used to drive the experiment.

In the external reference mode, the experiment includes some device, for example an
optical chopper, which generates a reference frequency that is applied to one of the
lock-in amplifier's external reference inputs. The instrument's reference channel
"locks" to this signal and uses it to measure the applied input signal, and indeed it is
this key capability that gives the instrument its name.

3.2.06 Virtual Reference Mode

If the instrument is operated in internal reference mode, measuring a signal which is
phase-locked to the internal oscillator, with the reference phase correctly adjusted,
then it will generate a stable non-zero X channel output and a zero Y channel output.
If, however, the signal is derived from a separate oscillator, then the X channel and
Y channel outputs will show variations at a frequency equal to the difference
between the signal and internal oscillator frequencies. If the latter is now set to be
equal to the former then in principle the variation in the outputs will cease, but in
practice this will not happen because of slow changes in the relative phase of the two
oscillators.

In the virtual reference mode, unique to SIGNAL RECOVERY lock-in amplifiers,
the Y channel output is used to make continuous adjustments to the internal

oscillator frequency and phase to achieve phase-lock with the applied signal, such
that the X channel output is maximized and the Y channel output zeroed.

If the instrument is correctly adjusted, particularly ensuring that the full-scale
sensitivity control is maintained at a suitable setting in relation to changes in the
signal level, then the virtual reference mode is capable of making signal recovery
measurements which are not possible with most other lock-in amplifiers.

3.3 Principles of Operation

3.3.01 Block Diagram

The model 7270 is a single compact instrument that uses digital signal processing
(DSP) techniques implemented in field-programmable gate arrays (FPGA), a
microprocessor and very low-noise analog circuitry to achieve its specifications. A
block diagram of the unit is shown in figure 3-1, and the sections that follow
describe how each functional block operates and the effect it has on the instrument's
performance.

Chapter 3, TECHNICAL DESCRIPTION

3-3

Figure 3-1, Model 7270 - Block Diagram

3.3.02 Signal Channel Inputs

The signal input amplifier can be set for either single-ended or differential voltage
mode operation, or single-ended current mode operation. In voltage mode a choice of
AC or DC coupling is available using an FET or bipolar input device. In current
mode a choice of two conversion gains is available to give optimum matching to the
applied signal. In both modes the input connector shells may be either floated via a
1 k resistor or grounded to the instrument's chassis ground. These various features
are discussed in the following paragraphs.

Input Connector Selection, A / -B / A - B

When set to the A mode, the lock-in amplifier measures the voltage between the
center and the shell of the A input BNC connector, whereas when set to the A-B
mode it measures the difference in voltage between the center pins of the A and B (I)
input BNC connectors.

The latter, differential, mode is often used to eliminate ground loops, although it is
worth noting that at very low signal levels it may be possible to make a substantial
reduction in unwanted offsets by using this mode with a short-circuit terminator on
the B (I) connector, rather than by simply using the A input mode.

The specification defined as the Common Mode Rejection Ratio, C.M.R.R.,
describes how well the instrument rejects common mode signals applied to the A and

Chapter 3, TECHNICAL DESCRIPTION

3-4

B (I) inputs when operating in differential input mode. It is usually given in decibels.
Hence a specification of > 100 dB implies that a common mode signal (i.e. a signal
simultaneously applied to both A and B (I) inputs) of 1 V will give rise to less than
10 µV of signal out of the input amplifier.

The input can also be set to the –B mode, in which case the lock-in amplifier
measures the voltage between the center and the shell of the B (I) input connector.
This extra mode effectively allows the input to be multiplexed between two different
single-ended signals, subject to the limitation that the user must allow for the signal
inversion (equivalent to a 180° phase-shift) which it introduces when reading the
outputs.

Input Connector Shell, Ground / Float

The input connector shells may be connected either directly to the instrument's
chassis ground or floated via a 1 k resistor. When in the float mode, the presence of
this resistor substantially reduces the problems that often occur in low-level lock-in
amplifier measurements due to ground loops.

Input Signal Selection, V / I

Although the voltage mode input is most commonly used, a current-to-voltage
converter may be switched into use to provide current mode input capability, in
which case the signal is connected to the B (I) connector. High impedance sources
(> 100 k) are inherently current sources and need to be measured with a low
impedance current mode input. Even when dealing with a voltage source in series
with a high impedance, the use of the current mode input may provide advantages in
terms of improved bandwidth and immunity from the effects of cable capacitance.

The converter may be set to low-noise or wide bandwidth conversion settings, but it
is worth noting that if the best possible performance is required a separate current
preamplifier, such as the SIGNAL RECOVERY models 181 or 5182, should be

considered.

3.3.03 Line Frequency Rejection Filter

Following the signal input amplifier there is an option to pass the signal through a
line frequency rejection filter, which is designed to give greater than 40 dB of
attenuation at the power line frequencies of 50 Hz or 60 Hz and their second
harmonics at 100 Hz and 120 Hz.

The filter uses two cascaded rejection stages with "notch" characteristics, allowing it
to be set to reject signals at frequencies equal to either of, or both of, the
fundamental and second harmonic of the line frequency.

Instruments are normally supplied with the line frequency filter set to 60 Hz with the
filter turned off. If the local line frequency is 50 Hz then the filter frequency should
be set to this value using the control on the Configuration menu (see section 2.3).

3.3.04 AC Gain and Dynamic Reserve

The signal channel contains a number of analog filters and amplifiers whose overall
gain is defined by the AC Gain parameter, which is specified in terms of decibels
(dB). For each value of AC Gain there is a corresponding value of the INPUT LIMIT
parameter, which is the maximum instantaneous (peak) voltage or current that can be
applied to the input without causing input overload, as shown in table 3-1 below.

Chapter 3, TECHNICAL DESCRIPTION

3-5

AC Gain (dB) INPUT LIMIT (mV)
0 2000
6 1000
12 500
18 250
24 125
30 62
36 31
42 16
48 8
54 4
60 2
66 1
72 0.5
78 0.25
84 0.125
90 0.062

Table 3-1, Input Limit vs. AC Gain

It is a basic property of the digital signal processing (DSP) lock-in amplifier that the
best demodulator performance is obtained by presenting as large a signal as possible
to the main analog-to-digital converter (ADC). Therefore, in principle, the AC Gain
value should be made as large as possible without causing the signal channel
amplifier or converter to overload. This constraint is not too critical however and the
use of a value one or two steps below the optimum value makes little difference.
Note that as the AC Gain value is changed, the demodulator gain (described later in
section 3.3.14) is also adjusted in order to maintain the selected full-scale sensitivity.

The full-scale sensitivity is set by a combination of AC Gain and demodulator gain.
Since the demodulator gain is entirely digital, changes in full-scale sensitivity which
do not change the AC Gain do not cause any of the errors which might arise from a
change in the AC Gain.

The user is prevented from setting an illegal AC Gain value, i.e. one that would
result in overload on a full-scale input signal. Similarly, if the user selects a full-scale
sensitivity that causes the present AC Gain value to be illegal, the AC Gain will
change to the nearest legal value.

In practice, this system is very easy to operate. However, the user may prefer to make
use of the AUTOMATIC AC Gain feature that gives very good results in most cases.
When this is active the AC Gain is automatically controlled by the instrument, which
determines the optimum setting based on the full-scale sensitivity currently being
used.

At any given setting, the ratio

ySensitivit Scale-Full

LimitInput

0.7DR 

represents the factor by which the largest acceptable sinusoidal interference input
exceeds the full-scale sensitivity and is called the Dynamic Reserve of the lock-in
amplifier at that setting. (The factor 0.7 is a peak-to-rms conversion).

Chapter 3, TECHNICAL DESCRIPTION

3-6

The dynamic reserve is often expressed in decibels, for which

))ratio a log(DR(as20dB)DR(in 

Applying this formula to the model 7270 at the maximum value of INPUT LIMIT
(2.0 V) and the smallest available value of FULL-SCALE SENSITIVITY (2 nV),
gives a maximum available dynamic reserve of about 1 × 109 or 180 dB. Figures of
this magnitude are available from any DSP lock-in amplifier but are based only on
arithmetical identities and do not give any indication of how the instrument actually
performs. In fact, all current DSP lock-in amplifiers become too noisy and inaccurate
for most purposes at reserves of greater than about 100 dB.

For the benefit of users who prefer to have the AC Gain value expressed in decibels,
the model 7270 displays the present value of Dynamic Reserve (DR) in this form, on
the input full-scale sensitivity control, for values up to 100 dB. Above 100 dB the
legend changes to “DR>100”.

3.3.05 Anti-Aliasing Filter

The signal then passes through an anti-aliasing filter to remove unwanted frequencies
which would cause a spurious output from the main ADC as a result of the sampling
process.

Consider the situation when the lock-in amplifier is measuring a sinusoidal signal of
frequency f

signal

Hz, which is sampled by the main ADC at a sampling frequency

f

sampling

Hz. In order to ensure correct operation of the instrument the output values

representing the f

signal

frequency must be uniquely generated by the signal to be

measured, and not by any other process.
However, if the input to the ADC has, in addition, an unwanted sinusoidal signal

with frequency f1 Hz, where f1 is greater than half the sampling frequency, then this
will appear in the output as a sampled-data sinusoid with frequency less than half the
sampling frequency, f

alias

= |f1 - nf

sampling

|, where n is an integer. This alias signal is

indistinguishable from the output generated when a genuine signal at frequency f

alias

is sampled. Hence if the frequency of the unwanted signal were such that the alias
signal frequency produced from it was close to, or equal to, that of the wanted signal
then it is clear that a spurious output would result.

For example, at the sampling frequency of 1.0 MHz then half the sampling frequency
is 500 kHz. If a signal of 40 kHz accompanied by an interfering signal of 950 kHz
was then applied, the output of the ADC would include a sampled-data sinusoid of
40 kHz (the required signal) and, applying the above formula, an alias signal of
50 kHz (i.e. |950 kHz - 1000 kHz|). f the signal frequency were now increased
towards 50 kHz then the output of the lock-in amplifier would increasingly be
affected by the presence of the alias signal and the accuracy of the measurement
would deteriorate.

To overcome this problem the signal is fed through the anti-aliasing filter which
restricts the signal bandwidth to an upper frequency of less than 250 kHz The filter is
a conventional elliptic-type, low-pass, stage, giving the lowest possible noise
bandwidth.

It should be noted that the dynamic range of a lock-in amplifier is normally so high
that practical anti-alias filters are not capable of completely removing the effect of a
full-scale alias. For instance, even if the filter gives 100 dB attenuation, an alias at

Chapter 3, TECHNICAL DESCRIPTION

3-7

the input limit and at the reference frequency will give a one percent output error
when the dynamic reserve is set to 60 dB, or a ten percent error when the dynamic
reserve is set to 80 dB.

In a typical low-level signal recovery situation, many unwanted inputs need to be
dealt with and it is normal practice to make small adjustments to the reference
frequency until a clear point on the frequency spectrum is reached. In this context an
unwanted alias is treated as just another interfering signal and its frequency is
avoided when setting the reference frequency.

A buffered version of the analog signal just prior to the main ADC is available at the
signal monitor (SIG MON) connector on the rear panel of the instrument; it may be
viewed on an oscilloscope to monitor the effect of the line frequency rejection and
anti-aliasing filters and signal-channel amplifiers.

3.3.06 Main Analog-to-Digital Converter

The analog signal is then routed to the main analog-to-digital converter, which runs
at a sampling rate of 1.0 MHz. The output from the converter feeds one of the two
demodulators, which uses DSP techniques to implement the digital multipliers and
output low-pass filters for each of the X and Y channels.

The ADC output also passes to the fast output curve buffer where it can be stored
both for direct user use (by downloading the data to a computer, or viewing it on the
user graphs), or to allow the power spectral density of the input signal to be
calculated. This uses a discrete Fourier transform, which in many ways is similar to a
fast Fourier transform (FFT), and the results of this calculation are shown on the
Spectral Display menu.

In dual reference and dual harmonic mode a second demodulator is active, and in
normal operation the input to this is also taken from the main ADC output.

Before discussing the demodulators and the output stages of the lock-in amplifier,
the reference channel, which provides the other input to the demodulators, will be
described.

3.3.07 Reference Channel Inputs

The 7270 provides two signal inputs for an external reference signal. The front panel
REF IN is a general-purpose input, designed to accept virtually any periodic
waveform with a 50:50 mark-space ratio and of suitable amplitude, while the rear­panel TTL REF IN is suitable for TTL-logic level input signals. Following the
trigger buffering circuitry the selected reference signal is routed to the reference
channel.

3.3.08 Reference Channel

The reference channel circuitry is responsible for implementing a phase-locked loop
to lock onto the selected external reference signal (when in external reference mode),
or processing signals from the internal oscillator (when in internal reference mode).
The reference channel generates a series of phase values, output at a rate of one
every 1 µs, which are used to drive the reference channel inputs of the two
demodulators.

In dual reference mode, the two references are selected from the three possible
options of internal, external front panel, and external rear panel inputs. The reference
circuit generates new phase values for each individual channel and sends these to the

Chapter 3, TECHNICAL DESCRIPTION

3-8

demodulators.
In single harmonic mode, the reference circuit generates the phase values of a

waveform at the selected harmonic of the reference frequency. Dual harmonic mode
operates in a similar way to dual reference mode, but in this case the reference circuit
generates phase values for both of the selected harmonics of the reference frequency.
Dual harmonic mode may therefore be used with either internal or external
references.

External Reference Mode

In external reference mode the reference is taken from one of two possible external
reference inputs.

Internal Reference Mode

With internal reference operation the reference circuit is free-running at the selected
reference frequency and is not dependent on a phase-locked loop (PLL), as is the
case in many other lock-in amplifiers. Consequently, the phase noise is extremely
low, and because no time is required for a PLL to acquire lock, reference acquisition
is immediate.

Both the signal channel and the reference channel contain calibration parameters that
are dependent on the reference frequency. These include corrections to the anti-alias
filter and to the analog circuits in the reference channel. In external reference
operation the processor uses a reference frequency meter to monitor the reference
frequency and updates these parameters when a change of about 2 percent has been
detected.

In all cases, a TTL logic signal at the current reference frequency is provided at the
REF MON connector on the rear panel.

3.3.09 Phase-Shifter

Each demodulator has a digital reference phase-shifter, allowing the phase values
being sent to the in-phase and quadrature multipliers to be adjusted to the required
value. If the reference input is a sinusoid applied the front panel REF IN connector,
the reference phase is defined as the phase of the X demodulation function with
respect to the reference input.

This means that when the reference phase is zero and the signal input to the
demodulator is a full-scale sinusoid in phase with the reference input sinusoid, the X
channel output of the demodulator is a full-scale positive value and the Y channel
output is zero.

The circuits connected to the REF IN connectors detect positive-going crossings of
the mean value of the applied reference voltage. Therefore when the reference input
is not sinusoidal, its effective phase is the phase of a sinusoid with a positive-going
zero crossing at the same point in time, and accordingly the reference phase is
defined with respect to this waveform. Similarly, the effective phase of a reference
input to the TTL REF IN socket is that of a sinusoid with a positive-going zero
crossing at the same point in time.

In basic lock-in amplifier applications the purpose of the experiment is to measure
the amplitude of a signal which is of fixed frequency and whose phase with respect
to the reference input does not vary. This is the scalar measurement, often
implemented with a chopped optical beam. Many other lock-in amplifier applications
are of the signed scalar type, in which the purpose of the experiment is to measure

Chapter 3, TECHNICAL DESCRIPTION

3-9

the amplitude and sign of a signal which is of fixed frequency and whose phase with
respect to the reference input does not vary apart from reversals of phase
corresponding to changes in the sign of the signal. A well-known example of this
situation is the case of a resistive bridge, one arm of which contains the sample to be
measured. Other examples occur in derivative spectroscopy, where a small
modulation is applied to the angle of the grating (in optical spectroscopy) or to the
applied magnetic field (in magnetic resonance spectroscopy). Double beam
spectroscopy is a further common example.

In this signed scalar measurement the phase-shifter must be set, after removal of any
zero errors, to maximize the X channel or the Y channel output of the demodulator.
This is the only method that will give correct operation as the output signal passes
through zero, and is also the best method to be used in an unsigned scalar
measurement where any significant amount of noise is present.

3.3.10 Internal Oscillator - General

The model 7270, in common with many other lock-in amplifiers, incorporates an
internal oscillator, which may be used to drive the experiment. However, unlike
many other instruments, the oscillator in the model 7270 is digitally synthesized with
the result that the output frequency is extremely accurate and stable. The oscillator
operates over the same frequency range as the lock-in amplifier, that is 1 mHz to

250.0 kHz. The oscillator signal is available at the OSC OUT connector.

3.3.11 Internal Oscillator - Update Rate

The direct digital synthesis (DDS) technique generates a waveform at the DAC
output, which is not a pure sinusoid, but rather a stepped approximation to one. This
is then filtered by the buffer stage, which follows the DAC, to reduce the harmonic
distortion to an acceptable level. The rate at which new values are sent to the DAC is

2.0 MHz.

3.3.12 Internal Oscillator - Frequency & Amplitude Sweeps

The internal oscillator output may be swept in both frequency and amplitude. In both
cases the sweeps take the form of a series of steps between starting and finishing
values. Frequency sweeps may use equal increment step sizes, giving a linear change
of frequency with time as the sweep proceeds, or may use step sizes proportional to
the present frequency, which produces a logarithmic sweep. The amplitude sweep
function offers only linear sweeps.

A special form of the frequency sweep function is used to acquire lock when the
instrument is operating in the virtual reference mode. When this "seek" sweep is
activated, the oscillator starts at a user-specified frequency, which should be just
below that of the applied signal, and increments until the calculated magnitude
output is greater than 50%. At this point the sweep then stops and the virtual
reference mode achieves lock, by continuously adjusting the internal oscillator
frequency to maintain the Y channel output at zero.

It is important to note that this type of phase-locked loop, unlike a conventional
edge-triggered type using a clean reference, does not automatically re-acquire lock
after it has been lost. Lock can be lost as a result of a signal channel transient or a
phase reversal of the signal, in which case it may be necessary to repeat the lock
acquisition procedure. However, if the measurement system is set up with sufficient
precautions, particularly ensuring that the full-scale sensitivity is maintained at a

Chapter 3, TECHNICAL DESCRIPTION

3-10

suitable setting in relation to the signal level, then the virtual reference mode is
capable of making signal recovery measurements which are not possible with other
lock-in amplifiers.

When virtual reference mode is in use, the signal at the OSC OUT connector is a
sinusoid which is phase-locked to the signal. Naturally, this cannot be used as a
source for the measurement.

3.3.13 Internal Oscillator - Voltage Control

The auxiliary ADC 1 input can be used to modulate the internal oscillator output
frequency or amplitude. Controls allow a quiescent amplitude and/or frequency to be
set, and a translation function (i.e. frequency and/or amplitude change per volt
change at the input) to be specified.

3.3.14 Demodulators - Dual Phase Multipliers

The function of each of the two demodulators is to multiply the digitized output of
the signal channel by digital representations of cosine and sine waves at the
demodulation frequency, to generate respectively the X and Y channel outputs. In
normal operation the demodulation frequency is at the internal or external reference
frequency, but when detecting at a harmonic of this then it is at some multiple, n (the
reference harmonic number) of it.

The demodulator outputs are digitally scaled to provide the demodulator gain
control. As discussed earlier in section 3.3.04 this gain is adjusted as the AC Gain is
adjusted to maintain the selected full-scale sensitivities.

In normal single reference mode the Demodulator 2 function is inactive, but it is
brought into operation when dual reference or dual harmonic modes are selected.

3.3.15 Demodulators - Output Filters

The outputs from the X channel and Y channel multipliers feed the X channel and Y
channel output low-pass filters, implemented as Finite Impulse Response (FIR)
stages with selectable 6 or 12 dB/octave slope (roll-off). Further filtering can be
carried out within the main output processor, to allow 18 and 24 dB/octave slopes.

In traditional audio terminology, a first-order low-pass filter is described as having a
slope of 6 dB per octave. This is because in the high frequency limit its gain is
inversely proportional to frequency (6 dB is approximately a factor of 2 in amplitude
and an octave is a factor of 2 in frequency). Similarly, a second-order low-pass filter
is described as having a slope of 12 dB per octave. These terms have become part of
the accepted terminology relating to lock-in amplifier output filters and are used in
the model 7270 to apply to the envelope of the frequency response function of the
digital finite impulse response (FIR) output filters. Accordingly the front-panel
control which selects the configuration of the output filters is labeled SLOPE and the
options are labeled 6, 12, 18, 24 dB/octave. Note that at the shorter time constant
settings the filter slope options are limited to 6 or 12 dB/octave.

The 6 dB/octave filters are not satisfactory for most purposes because they do not
give good rejection of non-random interfering signals, which can cause aliasing
problems as a result of the sampling process in the main ADC. However, the
6 dB/octave filter finds use where the lock-in amplifier is incorporated in a feedback
control loop, and in some situations where the form of the time-domain response is
critical. The user is recommended to use 12 dB/octave unless there is some definite

Chapter 3, TECHNICAL DESCRIPTION

3-11

reason for not doing so.
The filters are of the finite impulse response type with the averaging time of each

section being equal to double the nominal time constant. This in turn defines the
settling time following a step change in input signal as being 2  TC  n, where TC
is the time constant and n = 1 for 6 dB, 2 for 12 dB, 3 for 18 dB and 4 for 24 dB
slope settings. Hence, for example, the settling time after a step change at the input
when the TC is 100 ms and the slope is 12 dB/octave will be 400 ms.

When the reference frequency is below 10 Hz the synchronous filter option is
available. When selected, the actual time constant of the filters is not generally the
selected value TC, but the closest smaller value equal to an integer number of
reference cycles. If TC is greater than 1 reference cycle, then the time constant is
between TC/2 and TC. Where random noise is relatively small, synchronous filter
operation gives a major advantage in low-frequency measurements by enabling the
system to give a constant output even when the output time constant is equal to only
1 reference cycle.

3.3.16 Fast Curve Buffer

The fast curve buffer is a feature common to the models 7124, 7270, and 7230 lock­in amplifiers. It allows up to 100,000 sets of eight signals to be recorded at rates of
up to 1 MSa/s (1 µs per point), and supports a variety of trigger modes. The buffer
can also be used in a circular fashion, with new data overwriting the oldest data, to
allow capture of events up to the time of an applied trigger.

Although eight signals can be recorded, these include the X2 and Y2 outputs that are
only generated in the dual reference and dual harmonic modes, and the input to the
second demodulator. Hence unless one of these modes is selected, only five signals
are stored.

The signals that are stored are therefore as follows:

Single Reference/Virtual Reference Mode

SIG ADC This is the raw digitized data out of the main ADC
X(1) The X channel output from the first demodulator
Y(1) The Y channel output from the first demodulator
ADC 1 The digitized representation of the input to the auxiliary ADC1
ADC 2 The digitized representation of the input to the auxiliary ADC2

Dual Reference/Dual Harmonic Mode

DEMOD 2 The digital signal input to the second demodulator
X2 The X channel output from the second demodulator
Y2 The Y channel output from the second demodulator

Data that has been stored to the buffer can be displayed on the front panel graphical
displays or downloaded to a computer.

3.3.17 Main Output Processor - General

The same eight signals that pass to the fast curve buffer also pass to the main output
processor. This carries out further output filtering if required, generates derived
outputs, such as signal magnitude and phase, drives four DACs that in turn generate
analog representations of the instrument outputs, and implements the standard curve
buffer. These features are described in more detail in the following sections.

Chapter 3, TECHNICAL DESCRIPTION

3-12

3.3.18 Main Output Processor - Output Offset and Expand

Following the output filter, an output offset facility enables ±300% full-scale offset
to be applied to any or all of the X(1), Y(1), X2, Y2 output signals. The output
expand facility allows a ×10 expansion, performed by simple internal digital
multiplication, to be applied to the same output signals.

3.3.19 Main Output Processor - Vector Magnitude and Phase

The processor also implements the magnitude and signal phase calculation, which is
useful in many situations. If the input signal Vs(t) is a reference frequency sinusoid of
constant amplitude, and the output filters are set to a sufficiently long time constant,
the X and Y channel demodulator outputs are constant levels. The function
(X2 + Y2) is dependent only on the amplitude of the required signal Vs(t) (i.e. it is
not dependent on the phase of Vs(t) with respect to the reference input) and is
computed by the output processor, and made available as the magnitude output. The
phase angle between Vs(t) and the X demodulation function is called the signal
phase: this is equal to the angle of the complex quantity (X + jY ) (where j is the
square root of -1) and is also computed by the processor.

The magnitude and signal phase outputs are used in cases where phase is to be
measured, or alternatively where the magnitude is to be measured under conditions
of uncertain or varying phase.

One case of varying phase is that in which the reference input is not derived from the
same source as that which generates the signal, and is therefore not at exactly the
same frequency. In this case, if the input signal is a sinusoid of constant amplitude,
the X channel and Y channel demodulator outputs show slow sinusoidal variations at
the difference frequency, and the magnitude output remains steady.

However, the magnitude output has disadvantages where significant noise is present
at the outputs of the demodulator. When the required signal outputs (i.e. the mean
values of the demodulator outputs) are less than the noise, the outputs take both
positive and negative values but the magnitude algorithm gives only positive values.
This effect, sometimes called noise rectification, gives rise to a zero error which in
the case of a Gaussian process has a mean value equal to 0.798 times the combined
root-mean-square (rms) value of the X and Y demodulator noise. Note that unlike
other forms of zero error this is not a constant quantity which can be subtracted from
all readings, because when the square root of the sum of the squares of the required
outputs becomes greater than the total rms noise the error due to this mechanism
disappears.

A second type of signal-dependent error in the mean of the magnitude output occurs
as a result of the inherent non-linearity of the magnitude formula. This error is
always positive and its value, expressed as a fraction of the signal level, is half the
ratio of the mean-square value of the noise to the square of the signal.

Hence when the magnitude output is being used, the output filter time constants
should be set to give the required signal-to-noise ratio at the X channel and Y
channel demodulator outputs, rather than attempting to improve the signal-to-noise
ratio by averaging the magnitude output.

For analogous reasons, the magnitude function also shows signal-dependent errors
when zero offsets are present in the demodulator. For this reason, it is essential to
reduce zero offsets to an insignificant level (usually by the use of the Auto-Offset

Chapter 3, TECHNICAL DESCRIPTION

3-13

function) when the magnitude output is to be used.
Note that the majority of signal recovery applications are scalar measurements,

where the phase between the required signal and the reference voltage is constant
apart from possible phase reversals corresponding to changes in the sign of the
quantity being measured. In this situation the lock-in amplifier is used in the normal
X-Y mode, with the phase-shifter adjusted to maximize the X output and to bring the
mean Y output to zero. (Refer to section 3.3.26 for further information on the correct
use of the Auto-Phase function for this purpose.)

3.3.20 Main Output Processor - Noise Measurements

The noise measurement facility uses the output processor to perform a noise
computation on the X output of the demodulator. A noise buffer continuously
calculates the mean level of X, representing the measured output signal, by summing
the last n samples of the X output and dividing by n. The processor then calculates
the modulus of the difference between each X-output value and the mean value and
uses this figure to derive the noise. The displayed noise value is correct for input
noise where the amplitude distribution of the waveform is Gaussian, which is
normally the case. The indicated value (in V/Hz or A/Hz) is the square root of the
mean spectral density over the equivalent noise bandwidth defined by the setting of
the output filter time constant and slope.

When used for noise measurements, the available range of output time constants is
restricted to 500 µs to 10 ms inclusive, and the slope to 6 or 12 dB/octave. The
corresponding actual bandwidth for the present time constant and slope settings can
be found from the table 3-2 below, or by using the ENBW. command. In addition,
the synchronous time constant control is turned off, and the fast analog outputs mode
is turned on.

Time

Equivalent Noise Bandwidth at Output Filter Slope (Hz)

Constant

6 dB/octave

12 dB/octave

500 µs

500.00

333.33

1 ms

250.00

166.67

2 ms

125.00

83.33

5 ms

50.00

33.33

10 ms

25.00

16.67

Table 3-2, ENBW vs. Time Constant and Slope

The noise buffer length n can be set to 1, 1000, 2000, 3000 or 4000. Since new input
values to the buffer are supplied at a 1 kHz rate, these correspond to averaging times
of zero, 1 second, 2 seconds, 3 seconds, and 4 seconds respectively. Hence the
control on the Configuration Menu 1 that adjusts this buffer length is labeled Noise
Buffer Length and can be set to Off, 1 s, 2 s, 3 s or 4 s. Setting a shorter time means
that the system responds more quickly to changes in the mean X-output level, but the
noise reading itself exhibits more fluctuation. Conversely, the fluctuation can be
reduced by setting a longer time, but at the expense of increased settling time
following changes in the mean X-output level.

If a noise output (N calibrated in volts or amps per root hertz, or as a percentage of
full scale) is selected as one of the outputs on the Main Display or for conversion to
an analog signal for output to the DAC 1 to DAC 4 outputs, and the time constant is
not within the permitted range then a warning message is displayed on the screen.
Similarly, if a noise output value is read via the computer interfaces while the time
constant or slope are outside the permitted range, or if the synchronous time constant

Chapter 3, TECHNICAL DESCRIPTION

3-14

control is enabled, then the response will be -1. Since noise readings can only be zero
or positive, this negative number clearly indicates that the reading is invalid and
should be ignored.

In order to make noise measurements easier, the instrument includes a noise
measurement mode, activated by a control on the Configuration menu or by a
computer command. When this is turned on, the Main Display outputs are set to the
four types most commonly required, and the filter time constant, slope and
synchronous time constant setting are forced to values within the permitted ranges.
When turned off, these restrictions are removed.

When making noise measurements the user is strongly advised to use an oscilloscope
to monitor the signal at the SIG MON output on the rear panel as this is the best way
of ensuring that a random process is being measured rather than line pick-up or other
non-random signals.

3.3.21 Main Output Processor - Standard Curve Buffer

The output processor also operates the standard curve buffer, which is a 100,000
point memory that can be used for storage of selected instrument outputs as curves,
prior to their transfer to a computer via the computer interfaces. In addition to using
this function for the instrument outputs, such as the X channel and Y channel output
signals, it may also be used to store derived outputs and reference frequency
information.

Unlike the fast curve buffer, the available points are split between the number of
outputs to be stored, so that if for example X1 and Y1 outputs were selected, the
maximum recording length would be 50,000 points. Storage operates at rates of up to
1 kSa/s (1 ms per point).

3.3.22 Analog Outputs (DACs)

Earlier SIGNAL RECOVERY DSP lock-in amplifiers provided two different types
of analog outputs. Typically there were two outputs driven by DACs that in turn

were connected to the main instrument outputs, and separate auxiliary outputs,
essentially simply programmable DC voltages typically used as control signals for
the experiment. However this architecture created a number of restrictions. In
particular, in the dual modes it was not possible to get both X and Y outputs for both
demodulators when using short time constants, and in some cases it was necessary to
take the analog output from different connectors for different time constant ranges.

This design has been updated and so the 7270 is fitted with four general-purpose
DAC outputs, which can be driven from a variety of output signals, as well as the

traditional programmable “auxiliary DAC” signal, now referred to as the “User

DACs”.

Selection of the required outputs for the DACs is made on the DACs Menu. For each
DAC output there is a selection control, which is used to choose which instrument
output will be sent to the relevant DAC, or to specify that it will be a User DAC. If
the latter is selected then a further control allows the DAC voltage to be set.

Each DAC output is driven by a 16 bit converter operating at a raw update rate of
1 MSa/s, although depending on the output it is generating the actual update rate is
either 1 MSa/s or 1 kSa/s. When used for instrument outputs, full scale corresponds
to ±2.5 V, but the output remains linear to up to ±7.5 V; when used as a User DAC,
the range is ±10.000 V.

Chapter 3, TECHNICAL DESCRIPTION

3-15

3.3.23 Auxiliary Analog Inputs (ADCs)

The model 7270 incorporates four auxiliary ADC inputs offering a resolution of
1 mV in ±10.000 V. These converters may be used at slow sample rates for digitizing
slowly changing or DC signals which are associated with an experiment, such as
those generated by temperature and pressure transducers, so that they can be
incorporated into ratio calculations or transferred to a controlling computer. They
may also be used in conjunction with the instrument's fast curve buffer to form a
transient recorder operating at sample rates of up to 200 kHz (one channel) or
40 kHz (two channels).

3.3.24 Main Microprocessor - General

All functions of the instrument are under the control of a microprocessor, which in
addition drives the front-panel display, processes front-panel key operations and
supports the computer interfaces. This processor also supports the instrument's 8-bit
digital (TTL) programmable input/output port, which may be used for controlling
auxiliary apparatus or reading the status of external logic lines.

A particularly useful feature of the design is that only part of the controlling
firmware program code, which the microprocessor runs, is permanently resident in
the instrument. The remainder is held in a flash EEPROM and can be updated via the
USB or RS232 computer interface, using an Update Pack that can be downloaded
from the www.signalrecovery.com website.

3.3.25 Main Microprocessor - Spectral Display

In some cases it can be useful to determine the spectral power distribution of the
input signal. The model 7270 can do this, since when the Spectral Display menu is
selected, the output processor performs a discrete Fourier transform on the digitized
input signal and displays the resulting spectrum.

3.3.26 Main Microprocessor - Auto Functions

The microprocessor also controls the instrument's auto functions. These allow easier,
faster operation in most applications, although direct manual operation or special
purpose control programs may give better results in certain circumstances. During
operation of several of the auto functions, decisions are made on the basis of output
readings taken at a particular moment. Where this is the case, it is important for the
output time constant set by the user to be long enough to reduce the output noise to a
sufficiently low level so that valid decisions can be made and that sufficient time is
allowed for the output to settle.

The following sections contain brief descriptions of the auto functions.

Auto-Sensitivity

This function only operates when the reference frequency is above 1 Hz. A single
Auto-Sensitivity operation consists of decreasing the full-scale sensitivity range if
the magnitude output is greater than 90% of full-scale, or increasing the full-scale
sensitivity range if the magnitude output is less than 30% of full-scale. After the
Auto-Sensitivity function is called, Auto-Sensitivity operations continue to be made
until the required criterion is met.

In the presence of noise, or a time-varying input signal, it may be a long time before
the Auto-Sensitivity sequence comes to an end, and the resulting setting may not be
what is really required.

Chapter 3, TECHNICAL DESCRIPTION

3-16

Auto-Phase

In an Auto-Phase operation the value of the signal phase is computed and an
appropriate phase-shift is then introduced into the reference channel so as to bring
the value of the signal phase to zero. The intended result is to null the output of the Y
channel while maximizing the output of the X channel.

Any small residual phase can normally be removed by calling Auto-Phase for a
second time, after a suitable delay to allow the outputs to settle.

The Auto-Phase facility is normally used with a clean signal that is known to be of
stable phase. It usually gives very good results provided that the X channel and Y
channel outputs are steady when the procedure is called.

If a zero error is present on the outputs, such as may be caused by unwanted coupling
between the reference and signal channel inputs, then the following procedure should
be adopted:-

1) Remove the source of input signal, without disturbing any of the connections to
the signal input which might be picking up interfering signals from the reference
channel. In an optical experiment, for example, this could be done by shielding
the detector from the source of chopped light.

2) Execute an Auto-Offset operation, which will reduce the X channel and Y
channel outputs to zero.

3) Re-establish the source of input signal. The X channel and Y channel outputs
will now indicate the true level of input signal, at the present reference phase
setting.

4) Execute an Auto-Phase operation. This will set the reference phase-shifter to the
phase angle of the input signal. However, because the offset levels which were
applied in step 2 were calculated at the original reference phase setting, they will
not now be correct and the instrument will in general display a non-zero Y
channel output value.

5) Remove the source of input signal again.

6) Execute a second Auto-Offset operation, which will reduce the X channel and Y
channel outputs to zero at the new reference phase setting.

7) Re-establish the source of input signal.

This technique, although apparently complex, is the only way of removing the effect
of crosstalk, which is not generally in the same phase as the required signal.

Auto-Offset

In an Auto-Offset operation the X offset and Y offset functions are turned on and are
automatically set to the values required to give zero values at both the X and the Y
outputs. Any small residual values can normally be removed by calling Auto-Offset
for a second time after a suitable delay to allow the outputs to settle.

The primary use of the Auto-Offset is to cancel out zero errors which are usually
caused by unwanted coupling or crosstalk between the signal channel and the
reference channel, either in the external connections or possibly under some
conditions in the instrument itself. Note that if a zero error is present, the Auto­Offset function should be executed before any execution of Auto-Phase.

Chapter 3, TECHNICAL DESCRIPTION

3-17

Auto-Measure

This function only operates when the reference frequency is greater than 1 Hz. It
performs the following operations:

The AC GAIN value is adjusted to maximize the input to the main ADC without
causing overload, and the time-constant is set to an appropriate value for the present
reference frequency. An auto-sensitivity operation is then performed, followed by an
auto-phase.

The Auto-Measure function is intended to give a quick setting of the instrument
which will be approximately correct in typical simple measurement situations. For
optimum results in any given situation, it may be convenient to start with Auto­Measure and to make subsequent modifications to individual controls.

NOTE: The Auto-Measure function affects the setting of the AC Gain and AC
Gain Automatic controls during execution. Consequently, it may not operate
correctly if the AC Gain Automatic control is turned off. In this case, better results
will be obtained by performing Auto-Sensitivity followed by Auto-Phase functions.

Auto-Default

With an instrument of the design of the model 7270, where there are many controls
of which only a few are regularly adjusted, it is very easy to overlook the setting of
one of them. Consequently an Auto-Default function is provided, which sets all the
controls to a defined state. This is most often used as a rescue operation to bring the
instrument into a known condition when it is giving unexpected results. A listing of
the settings which are invoked by the use of this function can be found in
appendix D.

3.3.27 Main Microprocessor - User Settings

The non-volatile memory associated with the main microprocessor is used to store up
to eight complete instrument settings, which may be recalled or changed as required.
This makes it much easier for an instrument to be quickly configured for different
experiments.

This completes the description of the main functional blocks of the instrument.

3.4 General

3.4.01 Accuracy

When the demodulator is operating under correct conditions, the absolute gain
accuracy of the instrument is limited by the analog components in the signal channel,
and the absolute phase accuracy is limited by the analog components in both the
signal channel and the reference channel. The resulting typical accuracy is ±1.0
percent of the full-scale sensitivity and ±0.5 degree respectively.

3.4.02 Power-up Defaults

All instrument settings are retained when the unit is switched off. When the
instrument is switched on again the settings are restored but with the following
exceptions:-

a) The REMOTE parameter is set to zero (front-panel control enabled).
b) The curve buffer is cleared.
c) Any sweep that was in progress at switch-off is terminated.
d) The display is turned on.

Chapter 3, TECHNICAL DESCRIPTION

3-18

Front and Rear Panels

4.1 Front Panel

Figure 4-1, Model 7270 Front Panel Layout

Chapter 4

As shown in figure 4-1, the model 7270's front panel has four BNC connectors, a
320 × 240 pixel color LCD display panel, ten double and four single keys positioned
adjacent to the screen, four cursor-movement keys and a 12-button keypad. The
following sections describe the function and location of these items.

4.1.01 A and B (I) Signal Input Connectors

The A connector is the signal input connector for use in single-ended and differential
voltage mode. The B (I) connector is the signal input connector for use in differential
voltage mode (A-B) and for inverting single-ended voltage mode (-B mode). It is
also the signal input connector when the current input mode is selected.

When either input is overloaded the words INPUT OVERLOAD, in the top left-hand
corner of the screen, flash.

4.1.02 REF IN Connector

This is the general-purpose input connector for an external reference signal.

NOTE: If the best possible phase accuracy at low external reference frequencies is
required, then a TTL reference signal should be applied to the rear panel
TTL REF IN input instead.

When external reference mode is selected the word LOCKED appears in highlighted
text along the top edge of the screen. Under unlock conditions the word
UNLOCKED flashes.

4.1.03 OSC OUT Connector

This is the output connector for the internal oscillator.

4-1

Chapter 4, FRONT AND REAR PANELS

4.1.04 LCD Screen

This screen, the five pairs of keys on each side of it and the four keys under it are
used to adjust the instrument's controls and display the measured outputs, by the use
of a series of menus. The model 7270 is a very sophisticated instrument with many
features and consequently had the traditional approach of using one button per
control been adopted the front panel would have been very large. Adopting a menu­based control and display system, with the function of each key being dependent on
the displayed menu, gives a much cleaner design, with controls that need to be
changed only occasionally being hidden in normal use.

The ten pairs of keys on either side of the screen have the following functions,
depending on the displayed menu.

Function 1: To adjust the setting of a control.

If a control, such as time constant, full-scale sensitivity, or input coupling mode is
displayed on the screen then the adjacent key pair is used to adjust its setting.

Some controls, such as AC Gain and full-scale sensitivity, have only a limited range
of settings, and so the use of the and keys allows the required value to be
chosen with only a few key-presses. Other controls, such as the internal oscillator
amplitude and frequency, may be set over a wide range of values and to a high
resolution. In these cases a significant number of key-presses might be needed to set
the control to the required setting.

Adjustment of the latter type of control is made easier by any of the four methods
described below.

Keypad Data Entry

Pressing the single key below the lower left-hand corner of the screen marked
SET CONTROL followed by either side of the key adjacent to a control
requiring numerical entry activates the keypad. This is indicated by the
SET CONTROL key changing to ABORT KEYPAD and a small keypad icon
appearing adjacent to the selected control, as shown in figure 4-2.

Figure 4-2, Numerical Entry Keypad

Use the keypad to enter the required setting of the control. The numerical keys and
decimal point , sign and engineering exponent keys are self-explanatory,
while the (clear) key can be pressed at any time to clear any digits already
entered, which are shown in dimmed text next to the control.

4-2

Chapter 4, FRONT AND REAR PANELS

Once the required value is set, there are two choices:
If the (enter) key is pressed, then the value entered will be accepted as the new

control setting, the setting will be shown in normal, bright, text and the keypad icon
will be removed. If however the ABORT KEYPAD key is pressed, then the control
will be left at its previous setting and the keypad icon will be removed.

The following example should make this clear.

Example: Setting Internal Oscillator Frequency to 100.3 Hz

If the internal oscillator frequency is 1000 Hz and it is required to change it to

100.3 Hz, then press the front-panel SET CONTROL key followed by the key
adjacent to the Main Display OSC FREQUENCY control. The keypad icon will
appear and the keypad will be activated.

Press the keys corresponding to the digits “1”, “0”, “0”, and “3” to set the

oscillator frequency to 100.3 Hz. Finally, press the key to accept the value
and remove the keypad icon.

Active Cursor

In some cases it is useful to be able to quickly adjust a control in equal increments,
for example when monitoring the effect of changing the oscillator frequency about a
given value. This is easily done in the model 7270 using a control setting feature
known as Active Cursor operation.

A cursor can be placed under any digit of a displayed control where that control
requires numerical data entry. Thereafter, the keys adjacent to the control
increment or decrement the setting of that digit.

The positioning of the cursor can be done in one of two ways:

Using Cursor Movement Keys

The , , and cursor movement keys move the active cursor between all
digits to which it can apply on the relevant controls. The keys and keys adjust

the position of the cursor within a control while the and keys move the cursor
between controls.

This method of cursor positioning is very easy, but for compatibility with other
SIGNAL RECOVERY lock-in amplifiers, particularly the models 7220 and 7265,

the double key-press method of cursor positioning is also included.
Using the Double Keypress

Step 1 Simultaneously press both sides of the key adjacent to the control. In

the example shown in figure 4-3 the internal oscillator frequency will be
adjusted, since this is the control displayed adjacent to the keys. A cursor
appears under one of the displayed digits (see also figure 4-4).

Step 2 With the cursor visible, repeating step 1 causes the cursor to move to the left.

When the cursor reaches the most significant digit available (left end of
control setting) the next key-press returns the cursor to the least significant
digit (right end of control setting). Continue this action until the cursor is
under the required digit.

Step 3 Press the or key to change the digit to the required value.

4-3

Chapter 4, FRONT AND REAR PANELS

As an example of this operation, suppose that the oscillator frequency is

1000.000 Hz and it is required to change it to 1001.000 Hz. Simultaneously press

both and keys adjacent to the oscillator frequency display. Move the cursor,
by repeated double-key presses, until it is under the digit that is to be changed, in this
case the zero to the left of the decimal point. Then press the key to increment the
frequency by 1 Hz. The cursor will disappear as soon as the frequency is adjusted but
its position remains active until changed (see figure 4-4).

Figure 4-3, Active Cursor Activation

Figure 4-4, Active Cursor Operation

The double-key press action can also be performed with one finger by firmly
pressing the center of the key rocker that will deform to press both keys. The
active cursor can be used to set any particular digit. For example, if you only want to
adjust the reference phase in one degree steps leave the cursor over the first digit to
the left of the decimal point of the reference phase value.

Auto Repeat

If a or key adjacent to a control is pressed and held, then its action is
automatically repeated such that the control setting is incremented or decremented at
a rate approximately ten times faster than can be achieved by repeated manual key­presses.

Function 2: To Select a Menu or Sub-Menu

When the screen adjacent to a key pair displays a menu name, then pressing
either the or key selects that menu.

Function 3: To Execute a Pre-Programmed Function

When the screen adjacent to a key pair displays a pre-programmed function,
such as Auto-Measure or start frequency sweep, then pressing either the or
key executes that function.

4-4

Chapter 4, FRONT AND REAR PANELS

4.1.05 HELP Key

The model 7270 includes context-sensitive on-screen help. In many menus, pressing
the HELP key followed by a key adjacent to any displayed control or menu selection
provides information about that control or menu.

If information is required on other topics, then pressing HELP from the Main
Display accesses the help system, from which the required subject may be obtained
by pressing the relevant key.

To exit the help system and return to normal operation press the EXIT HELP key.

4.1.06 MENU Key

The model 7270 is controlled by a series of on-screen menus. When the Main
Display is shown the MENU key is used to access Main Menu 1, from which other
menus may be accessed.

The structure of the menus is fully discussed in chapter 5.

4.1.07 SELECT CONTROL Key

This key allows the user to select which four out of the possible twelve basic
instrument controls, including those such as full-scale sensitivity, time constant and
oscillator frequency, are shown on the Main Display and can therefore be directly
adjusted.

The selection operates as follows:
Step 1 Press the SELECT CONTROL key.
Step 2 Press either the or key next to the position to wish to allocate to a

particular control. Although five controls can be adjusted from the
Main Display, only the controls allocated to the lower four can be selected
by the user, since the topmost one is always used for the AC Gain control.

Each press of the or key changes the control allocated to the adjacent

position. Repeat until the control you require is shown.

Step 3 Press the SELECTION COMPLETE key to return to the Main Display.

4-5

Chapter 4, FRONT AND REAR PANELS

4.2 Rear Panel

Figure 4-5, Model 7270 Rear Panel Layout

As shown in figure 4-5, the line power switch, line power voltage selector, two
RS232 connectors, USB, LAN and digital I/O port connectors, preamplifier power
connector, and fourteen BNC signal connectors are mounted on the rear panel of the
instrument. Brief descriptions of these are given in the following text.

4.2.01 Line Power Switch

CAUTION: The model 7270 may be damaged if the line voltage is set for 110 V
AC operation and it is turned on with 220 V AC applied to the power input
connector. Please ensure that the line voltage selector is set to the correct line
voltage before switching on.

Press the end of the switch marked I to turn on the instrument's power, and the other
end marked O to turn it off.

4.2.02 Line Power Input Assembly

This houses the line voltage selector and line input fuse. To check, and if necessary
change, the fuse or line voltage see the procedure in section 2.1.05.

4.2.03 DIGITAL I/O Connector

This connector provides eight TTL lines, each of which can be configured as an
input or output. When set as an output, the line can be set high or low by the use of
the Digital Port menu or via the computer interfaces, and when set as an input, the
applied logic state can be read.

The port is most commonly used for controlling auxiliary apparatus, such as lamps,
shutters and heaters, and reading status signals from auxiliary equipment. Pinouts for
this connector are given in appendix B.

4.2.04 USB Connector

This connector allows the instrument to be connected to a PC-style computer running
a Windows operating system via the USB bus. The instrument supports connection at

4-6

Chapter 4, FRONT AND REAR PANELS

both Full Speed and Hi-Speed (USB 2.0) protocols.

4.2.05 LAN Connector

This connector allows the instrument to be connected to a 100-BaseT or 10-BaseT
network for remote control. The IP address can be set to a static value or be set to
automatic, when the instrument will accept an address allocated by a DHCP server
on the network, using the controls on the Ethernet Setting Menu - see section 5.

4.2.06 RS232 Connector

This 9-pin D type RS232 interface connector implements pins 1, 2, 3 and 7 (Earth
Ground, Transmit Data, Receive Data, Logic Ground) of a standard DTE interface.
To make a connection to a PC-compatible computer, it is normally sufficient to use a
three-wire cable connecting Transmit Data to Receive Data, Receive Data to
Transmit Data, and Logic Ground to Logic Ground. Appendix C shows the
connection diagrams of cables suitable for computers with 9-pin and 25-pin serial
connectors. Pinouts for this connector are given in appendix B.

4.2.07 AUX RS232 Connector

This connector is used to link other compatible SIGNAL RECOVERY equipment
together in a "daisy-chain" configuration. Up to an additional 15 units can be

connected in this way. Each unit must be set to a unique address (see section 5.3.17).
Pinouts for this connector are given in appendix B.

4.2.08 PRE-AMP POWER Connector

This connector supplies ±15 V at up to 100 mA and can be used for powering
optional remote preamplifiers available from SIGNAL RECOVERY. Pinouts for

this connector are given in appendix B.

4.2.09 ADC TRIG IN Connector

This connector accepts a TTL-compatible input and can be used for triggering the
digitization of the voltages present at the auxiliary analog-to-digital converters
(ADCs). The input operates on the positive edge only.

4.2.10 ADC 1, ADC 2, ADC 3 and ADC 4 Connectors

The input voltages at these connectors may be digitized using the auxiliary ADCs
and read either from the front panel or by the use of a computer command. The input
voltages are sampled and held when the ADC is triggered, and several different
trigger modes are available. These modes can be set either from the front panel or by
using a remote computer command. The input voltage range is ±10.000 V and the
resolution is 1 mV.

4.2.11 REF MON Connector

The signal at this connector is a TTL-compatible waveform synchronous with the
reference. This output monitors correct reference channel operation but its polarity is
not uniquely defined so that it does not necessarily show the correct phase
relationship with the SIG MON output.

4.2.12 TTL REF IN Connector

This connector is provided to allow TTL-compatible pulses to be used as the
reference input, if the best possible phase accuracy at low external reference
frequencies is required, when it usually gives better results than the REF IN

4-7

Chapter 4, FRONT AND REAR PANELS

connector on the front panel.

4.2.13 TRIG IN Connector

This connector accepts a TTL-compatible input and can be used for triggering data
acquisition to the internal curve buffer. The input can be set to respond to positive or
negative going edges.

4.2.14 TRIG OUT Connector

This connector can be set to generate a TTL-compatible output pulse, for example to
indicate when date is being sampled when using the internal curve buffer.

4.2.15 DAC 1, DAC 2, DAC 3, and DAC 4 Connectors

There are four digital-to-analog converter (DAC) output connectors. The output
voltages at these connectors can be configured to represent instrument outputs (e.g.
X and Y channel outputs) or be used as general purpose programmable voltages.
When used as instrument outputs the full-scale output range is ±2.500 V, with the
ability to operate linearly at up to three times this range. When used as general
purpose outputs, the output range is ±10.000 V

4.4.07 SIG MON Connector

The signal at this connector is that immediately prior to conversion by the main
ADC.

4-8

Front Panel Operation

Chapter 5

5-1

5.1 Introduction

This chapter describes how to operate the model 7270 using the front panel controls,
and discusses its capabilities when used in this way. Chapter 6 provides similar
information for when the unit is operated remotely using one of the computer
interfaces.

It is assumed that readers are already familiar with the use of the front panel and

keys, but if not then they should refer to the detailed description of their

operation given in chapter 4.
The model 7270 uses a flexible, menu-based, control structure that allows many

instrument controls to be adjusted from the front panel with only a few key presses.
Furthermore this design makes it very easy to introduce new features or improve
existing ones without the restrictions which would result from a fixed front panel
layout.

The instrument may be operated in one of four modes, as follows:-

Single Reference

This is the normal operating mode of the unit, where it functions as a conventional
dual phase lock-in amplifier. It includes both internal and external reference modes
and provides detection either at the reference frequency or one harmonic of it.

Virtual Reference

Virtual reference mode is an extension of internal single reference mode operation,
where the Y channel output is used to make continuous adjustments to the internal
oscillator frequency and phase. The effect is to achieve phase-lock with the applied
signal such that the X channel output is maximized and the Y channel output is
zeroed. Virtual reference mode operation is only possible with signals at frequencies
between 100 Hz and 250 kHz.

Dual Reference

In dual reference mode the model 7270 can make simultaneous measurements at two
different reference frequencies. If one is an external and the other the internal
oscillator, then both can be of any frequency from 0.001 Hz to 250 kHz.
Alternatively, if two external reference frequencies are used then one of them is
restricted to a maximum frequency of 3.0 kHz.

Tandem Demodulation

Some experiments, such as pump-probe optical work, generate an amplitude
modulated carrier signal. The carrier signal is at one, typically higher, frequency, and
the modulation at another, lower, frequency. The required measurement is the
amplitude and/or phase of the modulation.

Conventionally, lock-in detection of this signal would require two instruments. The
first would be run at the carrier frequency with its output filters set to a sufficiently
short time constant to allow the amplitude modulation to pass. Hence the X-channel
analog output would be the modulation that was to be measured; to achieve this the
signal would then be passed to the signal input of a second lock-in amplifier running
at the modulation frequency.

The built-in Tandem Demodulation mode, which is essentially the same as dual

Chapter 5, FRONT PANEL OPERATION

5-2

reference mode but with the input to the second set of demodulators taken as the X­channel output from the first, allows the 7270 to make this measurement in a single
instrument.

Dual Harmonic

Dual harmonic mode allows the simultaneous measurement of two different
harmonics of the input signal.

The sections that follow describe the menus as they appear when the unit is being
used in single reference mode. The menus range from the Main Display, used most
of the time for instrument control and display of data, through to those menus
accessing controls which typically only need changing occasionally.

The menus for the other three operating modes are then described, since in some
cases these differ from those used in single reference mode to accommodate the
additional controls and displays that are needed.

5.2 Menu Structure

Figure 5-1 shows the basic structure of the main instrument control menus which, it
will be seen, has a hierarchical, or "tree", structure.

Figure 5-1, Main Menu Structure

In the diagram, although not in the rest of this manual, the following syntax is used:­The menus are shown as boxes, with menu names in gothic typeface, e.g.
MAIN MENU, and arrows on the lines connecting the menus and the text adjacent to
them indicate the keys which need to be pressed to move between menus.

The following examples should make this clear.
To access Main Menu 1 from the Main Display, press the Menu key in the lower

right-hand corner of the screen.
To access the Signal Channel menu from the Main Display, press the Menu key

Chapter 5, FRONT PANEL OPERATION

5-3

followed by the Signal Channel key shown in Main Menu 1; to return to
Main Menu 1 press the Previous Menu key on the Signal Channel menu.

Note that all menus provide a Previous Menu key allowing the user to return one step
up the menu "tree". In addition, when in any menu, pressing the Main Display key on
the front panel provides a direct return route to the Main Display.

Some menus, such as the Oscillator menu, have further sub-menus that are discussed
later. These have been omitted from figure 5-1 for the sake of clarity.

5.3 Menu Descriptions - Single Reference Mode

5.3.01 Main Display

Figure 5-2, Main Display - Single Reference Mode

The Main Display always appears on power-up and is similar to that shown in
figure 5-2 above. It is divided into two sections by a single vertical line. Five
instrument controls appear on the left-hand side, of which the topmost one, AC Gain,
is always displayed, whereas the other four are user-specified using the Control
Selection menu, discussed later in section 5.3.02. On the right-hand side, four
instrument outputs are displayed in one of the three following formats:-

a) Two large numeric and two bar-graphs
b) Four bar-graphs
c) Four large numeric displays
The display mode is selected via the center key pair of the five keys to the

right of the screen, with each key press changing the mode. In any given display
mode, the choice of the output that will actually be shown in each of the four
positions is made using the corresponding right-hand keys. In single reference
mode, there are thirteen possible outputs to choose from for each numeric display,
with nine choices for the bar-graph displays, as listed in table 5-1.

Chapter 5, FRONT PANEL OPERATION

5-4

Output Description
Numeric Displays only:

R% Resultant (Magnitude) output as a percentage of full-scale sensitivity
N% Noise output as a percentage of full-scale sensitivity
° Phase output in degrees
X% X channel output as a percentage of full-scale sensitivity

Output Description
Numeric Displays only (continued):

Y% Y channel output as a percentage of full-scale sensitivity
R Resultant (Magnitude) output in volts or amps

Numeric and Bar-Graph Displays:

X X channel output in volts or amps
Y Y channel output in volts or amps
N Noise output in volts or amps per root hertz
ADC1 ADC1 input, ±10.000 V full-scale
ADC2 ADC2 input, ±10.000 V full-scale
ADC3 ADC3 input, ±10.000 V full-scale
ADC4 ADC4 input, ±10.000 V full-scale

Bar-Graph Displays only:

MAG Resultant (Magnitude) output in volts or amps
PHA° Phase output in degrees

Table 5-1, Output Display Choices - Single Reference Mode

The instrument provides a means of switching quickly between the following pairs of
outputs, simply by pressing simultaneously both ends of the keys adjacent to
their description:-

X %fs  X volts or amps
Y %fs  Y volts or amps
R %fs  R volts or amps
Noise %fs  Noise volts/Hz or amps/Hz

In the center right-hand section of the Main Display the current reference frequency,
as measured by the reference frequency meter, is shown, together with the current
levels of X and Y output offsets.

Warning Indicators
Along the top edge of the Main Display are four warning indicators. Input Overload
and Output Overload are normally shown in dimmed text, but in the event of an
overload occurring, flash on and off. Similarly, Locked appears in green text when
the instrument is locked to a suitable reference, and changes to Unlock in red
flashing text when the reference channel is unlocked.. Remote is normally shown in
dimmed text but appears in bright text when the instrument is being operated via one
of the computer interfaces.

Controls

AC GAIN

The AC Gain control is always displayed in the top left-hand corner of the
Main Display. If the automatic AC Gain control is turned off (using the Signal
Channel menu - see section 5.3.04), then this control allows the AC Gain to be

Chapter 5, FRONT PANEL OPERATION

5-5

adjusted from 0 dB to 90 dB in 6 dB steps, although not all settings are available at
all full-scale sensitivity settings. If the automatic control is turned on, then the
control cannot be adjusted, but the present value of AC Gain is still displayed. In
either mode, changing the full-scale sensitivity may result in a change to the AC
Gain.

To obtain the best accuracy, use the highest value of AC Gain that is possible
without causing signal input overload, indicated by the words Input Overload in the
top left-hand corner of the screen flashing on and off. The Input Limit value,
displayed immediately under the AC Gain control, is the largest value of rms signal
that may be applied without causing signal overload.

5.3.02 Control Selection Menu

The four user-selectable controls on the Main Display may be chosen from those
available by pressing the Set Control key. The Control Selection menu appears, as
shown in figure 5-3.

Figure 5-3, Control Selection Setup Menu

Press the keys adjacent to each of the four control descriptions on the left­hand side until the required controls are selected. Note that it is not possible to
display the same control in more than one position simultaneously.

The available controls have the following functions:-

SENSITIVITY

When set to voltage input mode, using the Signal Channel menu, the instrument's
full-scale voltage sensitivity may be set to any value between 2 nV and 1 V in a
1-2-5 sequence.

When set to current input mode, using the Signal Channel menu, the instrument's
full-scale current sensitivity may be set to any value between 2 fA and 1 µA (wide
bandwidth mode, or 2 fA and 10 nA (low-noise mode), in a 1-2-5 sequence.

The number reported after the letters DR is the instrument's Dynamic Reserve,
expressed in decibels, as calculated by the following equation:-

10

2

DR 20 log ACGain (in dB)

SEN



  




Chapter 5, FRONT PANEL OPERATION

5-6

For example, if AC Gain = 12 dB and SEN = 2 mV then

10

2

DR 20 log 12

0.002



  




DR = 48 dB

TIME CONSTANT

The time constant of the output filters is set using this control. When the Time
Constant Mode on the Output Filters menu (see section 5.3.06) is set to Fast, the
range is 10 µs to 100 ks in a 1-2-5 sequence; when the Time Constant Mode is set to
Normal the minimum time constant is 5 ms.

REF PHASE

This control allows the reference phase to be adjusted over the range -180° to +180°
in 1 m° steps. Adjustment is faster using the Keypad or Active Cursor controls - see
section 4.1.04.

REF PHASE ±90°

This control allows the reference phase to be adjusted in steps of ±90°.
X OFFSET and Y OFFSET

These are the manual X channel and Y channel output offset controls. The offset
levels set by these controls, which can be any value between -300% and +300% in

0.01% steps, are added to the X channel or Y channel outputs when the X channel or

Y channel offsets are switched on using the Output Channels menu. Adjustment is
faster using the Keypad or Active Cursor controls - see section 4.1.04.

The values are set automatically by the Auto-Offset function. Note that the Auto­Offset function automatically switches on both X and Y channel output offsets.

REF HARMONIC

This control sets the harmonic of the applied reference frequency, either internal or
external, at which the lock-in amplifier's reference channel operates, in the range 1st
(fundamental mode) to 127th. For example, if the control is set to 2nd and a
reference signal of 1.000 kHz is applied, the instrument will measure signals at its
input at a frequency of 2.000 kHz.

RELOCK EXT. REFERENCE

The 7270 includes frequency-dependent calibration parameters. When operating in
Internal reference mode the correct parameters can be chosen because the reference
frequency is equal to the specified oscillator frequency. In External reference mode
the applied frequency is measured, and the measured value is used to select the
correct parameters. If, however, the external reference frequency drifts or changes
with time then the lock-in amplifier may need to use different calibration parameters.
It therefore includes an automatic algorithm that detects significant changes in
reference frequency, and if these occur, updates all the frequency-dependent
calibration values.

Pressing this key has the effect of manually updating these calibration parameters.
This should be done when operating in External reference mode after each
intentional change to the applied reference frequency.

AUTO MEASURE

This control allows an auto-measure cycle to be initiated directly from the main

Chapter 5, FRONT PANEL OPERATION

5-7

display. It is equivalent to accessing the function via the Auto Functions Menu - see
section 5.3.14

AUX DAC 1, AUX DAC 2, AUX DAC 3, and AUX DAC 4
These two controls set the voltage appearing at the DAC1, DAC2, DAC3, and
DAC4 output connectors on the rear panel to any value between +10 V and -10 V
with a resolution of 1 mV, assuming that the corresponding output is set to the
USER DAC mode on the DAC menu - see section. Adjustment is faster using the
Keypad or Active Cursor controls - see section 4.1.04.

OSC FREQUENCY

The frequency of the instrument's internal oscillator may be set, using this control, to
any value between 0.001 Hz and 250.000 kHz with a 1 mHz resolution. Adjustment
is faster using the Keypad or Active Cursor controls - see section 4.1.04.

OSC AMPLITUDE

This control sets the amplitude of the signal at the 7270’s OSC OUT connector to
any value between 1 mV and 5 V rms. Adjustment is faster using the Keypad or
Active Cursor controls - see section 4.1.04.

Once the required controls have been selected, press the Selection Complete key on
the front panel to return to the Main Display.

5.3.03 Main Menu 1

When in the Main Display, press the Menu key once to access Main Menu 1, which
is shown in figure 5-4.

Figure 5-4, Main Menu

Main Menu 1 is used to access all of the remaining instrument controls via a series of
sub-menus, which are selected simply by pressing the key adjacent to the required
menu. These sub-menus are described in the following sections.

5.3.04 Signal Channel Menu 1

When Main Menu 1 is displayed, pressing a key adjacent to the Signal Channel item
accesses the Signal Channel Menu 1, which is shown in figures 5-5 and 5-6.

Chapter 5, FRONT PANEL OPERATION

5-8

Figure 5-5, Signal Channel Menu 1 - Voltage Input Mode

Figure 5-6, Signal Channel Menu 1 - Current Input Mode

The Signal Channel Menu 1 has four controls affecting the instrument's signal input
channel, and a key for accessing Signal Channel Menu 2. Changes to the setting of
these controls can be made by using the adjacent keys, with the currently
active selection being shown in highlighted text.

INPUT COUPLING

The input coupling can be set as follows:-

AC

The signal channel is AC coupled at the input with a lower-frequency –3 dB
cut-off of < 0.2 Hz, although it is recommend to use DC coupling at frequencies
< 1.0 Hz for best results. If phase (as opposed to magnitude) readings are
important then DC coupling should be used at higher frequencies, since no
correction is made for amplitude or phase changes caused by the AC coupling
capacitor. Bipolar mode cannot be AC coupled.

DC

The signal channel is DC coupled

Chapter 5, FRONT PANEL OPERATION

5-9

INPUT SHELL

The input connector shells can optionally be floated or connected to chassis ground
as follows:-

GROUND

The shells of the A and B (I) connectors are connected directly to chassis
ground.

FLOAT

The shells of the A and B (I) connectors are connected to chassis ground via a
1 k resistor.

INPUT MODE
This control sets the preamplifier input configuration to either voltage or current
mode, and also changes the function of the INPUT CONNECTOR control.

INPUT CONNECTOR
In voltage input mode, as shown in figure 5-5, this control has four settings:-

A

The signal channel input is a single-ended voltage input to the BNC connector on
the front panel marked A.

–B

The signal channel input is an inverting single-ended voltage input to the BNC
connector on the front panel marked B (I).

A-B

In this setting the signal channel input is a differential voltage input connected to
the BNC connectors on the front panel marked A and B (I).

OFF

The input is disconnected in this setting.

In current input mode, shown in figure 5-6, this control has two settings:­ WIDE-B/W (Wide Bandwidth Converter)

In this setting the signal channel input is a single-ended current input connected
to the BNC connector on the front panel marked B (I), and uses the wide
bandwidth current-to-voltage converter.

LO-NOISE (Low Noise Converter)

In this setting the signal channel input is a single-ended current input connected
to the BNC connector on the front panel marked B (I), and uses the low-noise
current-to-voltage converter.

Signal Channel Menu 2

Pressing a key adjacent to the Signal Channel Menu 2 item accesses the Signal
Channel Menu 2, which is shown in figure 5-7.

Chapter 5, FRONT PANEL OPERATION

5-10

Figure 5-7, Signal Channel Menu 2

The Signal Channel Menu 2 has four controls affecting the instrument's signal input
channel, and a key for accessing Signal Channel Menu 1. Changes to the setting of
these controls can be made by using the adjacent keys, with the currently
active selection being shown in highlighted text.

INPUT DEVICE

This control selects the type of input device used for the signal channel connections.
FET (FET input device)

Uses a FET as the input device, for which case the input impedance is 10 M.
This is the usual setting.

BIPOLAR (Bipolar input device)

Uses a bipolar device in the input stage, for the lowest possible voltage input
noise. In this case the input impedance is 10 k. Note that this selection is not
possible when using the AC-coupled input modes.

LINE NOTCH FILTER
This control selects the mode of operation of the line frequency rejection filter and
offers four possible settings out of the seven described in the following table:-

Legend Function

OFF Line filter inactive
50 Enable 50 Hz notch filter
60 Enable 60 Hz notch filter
100 Enable 100 Hz notch filter
120 Enable 120 Hz notch filter
50/100 Enable 50 and 100 Hz notch filters
60/120 Enable 60 and 120 Hz notch filters

The filter frequencies available (i.e. 50/100 Hz or 60/120 Hz) depend on the setting
of the LINE FREQUENCY control on the Configuration Menu - see section 5.3.15

AUTO AC GAIN

This control selects whether or not the Automatic AC Gain function is active. As
discussed in section 3.3.04, the correct adjustment of the AC Gain in a DSP lock-in

Chapter 5, FRONT PANEL OPERATION

5-11

amplifier is necessary to achieve the best results. This control allows the user to
select whether this adjustment is carried out automatically or remains under manual
control.

OFF

In this setting the AC Gain may be manually adjusted from the Main Display.

ON

In this setting the AC Gain value is automatically selected by the instrument,
depending on the full-scale sensitivity. In the mid-range full-scale sensitivity
ranges the resulting dynamic reserve is between 20 and 26 dB.

DEMOD2 SOURCE

When the demodulators are configured for dual reference operation, this control
selects the signal source for the second set of demodulators

SIGADC

In this setting the signal input to the second set of demodulators is taken from the
main signal channel ADC. This is the normal mode of operation. In dual
reference or dual harmonic mode this means that the two signals being measured
are applied to the same input connector and share a common path through the
main ADC.

ADC1

In this setting the signal input to the second set of demodulators is taken from the
auxiliary input ADC, ADC1. The full-scale sensitivity is adjustable over the
normal range of 1 V to 2 nV, but because there is no AC Gain, the effective
maximum sensitivity is 1 mV.

DEMOD1 (Tandem demodulator mode)

In this setting the signal input to the second set of demodulators is taken from the
X-channel output of the first set of demodulators

Signal Channel Menu 1

Pressing a key adjacent to the Signal Channel Menu 1 item accesses the Signal
Channel Menu 1; pressing the Previous Menu key returns control to Main Menu 1.

5.3.05 Reference Channel Menu

When Main Menu 1 is displayed, pressing a key adjacent to the Reference Channel
item accesses the Reference Channel menu, which is shown in figure 5-8.

Chapter 5, FRONT PANEL OPERATION

5-12

Figure 5-8, Reference Channel Menu

The Reference Channel menu has five controls affecting the instrument's reference
channel. Changes to the setting of these controls can be made by using the adjacent

keys.

REF SOURCE

This control allows selection of the source of reference signal used to drive the
reference circuitry, and has three settings:-

INT

The lock-in amplifier's reference is taken from the instrument's internal
oscillator. Note that this setting gives the best phase and gain measurement
accuracy under all operating conditions, and it is always to be preferred, if
possible, to design the experiment so that the lock-in amplifier acts as the source
of the reference signal.

EXT–rp

In this setting the reference channel is configured to accept a suitable external
reference source applied to the rear panel TTL REF IN input connector

EXT–fp

In this setting the reference channel is configured to accept a suitable external
reference source applied to the front panel REF IN input connector

REF HARMONIC

This control allows selection of the harmonic of the reference frequency at which the
lock-in amplifier will detect. It can be set to any value between 1st and 127th, but
most commonly is set to 1st. Note that the "2F" setting commonly found on other
lock-in amplifiers corresponds to setting this control to 2nd.

REF PHASE

This control allows the reference phase to be adjusted over the range -180° to +180°
in 1 m° steps.

REF PHASE ±90°

This control allows the reference phase to be adjusted in steps of ±90°.

Chapter 5, FRONT PANEL OPERATION

5-13

AUTO PHASE

In an Auto-Phase operation the value of the signal phase with respect to the reference
is computed and an appropriate phase-shift is then introduced into the reference
channel so as to bring the difference between them to zero. The intended result is to
null the output of the Y channel while maximizing the output of the X channel.

Pressing the Previous Menu key returns control to Main Menu 1.

5.3.06 Output Filters Menu

When Main Menu 1 is displayed, pressing a key adjacent to the Output Filters item
accesses the Output Filters menu, which is shown in figure 5-9.

Figure 5-9, Output Filters Menu

The Output Filters menu has four controls affecting the instrument's main X and Y
channel output filters. Changes to the setting of these controls can be made by using
the adjacent keys.

TIME CONSTANT

This control, which duplicates the Main Display TIME CONSTANT control, is used to
set the time constant of the output filters.

SLOPE

The roll-off of the output filters is set, using this control, to any value from 6 dB to
24 dB/octave, in 6 dB steps. Note that there are some restrictions in that it is not
possible to select 18 or 24 dB/octave settings at Time Constants of 500 ms or shorter
when using the fast time constant mode..

SYNC TIME CONSTANT

This control has three settings, as follows:-

OFF

In this setting, which is the normal mode, time constants are not related to the
reference frequency period.

ON

In this setting, the actual time constant used is chosen to be some multiple of the
reference frequency period, giving a much more stable output at reference
frequencies below 10 Hz for the Normal time constant mode (below 1 kHz for

Chapter 5, FRONT PANEL OPERATION

5-14

the Fast time constant mode) than would otherwise be the case. In this setting,
the actual time constant will not change if the reference changes. Note that,
depending on the reference frequency, output time constants shorter than 100 ms
cannot be used.

AUTO

This is the same as the ON setting except that if the reference changes the time
constant will be recalculate to the nearest legal synchronous setting.

TIME CONSTANT MODE

This control has two settings, as follows:-

NORMAL

In this setting, the analog outputs are derived from the output processor. The
update rate is always 1 kHz, the output filter time constant can be set to values
between 5 ms and 100 ks in a 1-2-5 sequence, and all four output filter slope
settings are available.

FAST

In this setting, the analog outputs are derived directly from the core FPGA
running the demodulator algorithms. The update rate is increased to 1.0 MHz
when the time constant is set to any value from 10 µs to 500 ms but remains at
1 kHz for longer time constants. The output filter slope is restricted to either 6 or
12 dB/octave.

Pressing the Previous Menu key returns control to Main Menu 1.

5.3.07 Output Offset & Expand Menu

When Main Menu 1 is displayed, pressing a key adjacent to the Offset & Expand
item accesses the Offset & Expand menu, which is shown in figure 5-10.

Figure 5-10, Output Offset & Expand Menu

The Offset & Expand menu has five controls affecting the instrument's X channel
and Y channel outputs. Changes to the setting of these controls can be made by using
the adjacent keys, with the currently active selection being highlighted.

X OFFSET and Y OFFSET
These controls, which duplicate the Main Display X OFFSET and Y OFFSET controls,

Chapter 5, FRONT PANEL OPERATION

5-15

allow manual adjustment of the X channel and Y channel output offsets. The offset
level set by the controls, which can be any value between -300% and +300% in

0.01% steps, is added to the X channel or Y channel output when the X channel or Y
channel offset is switched on. Adjustment is faster using the Keypad or Active
Cursor controls - see section 4.1.04.

The values are set automatically by the Auto-Offset function, which also switches on
both X channel and Y channel output offsets.

OFFSET STATUS

This control allows the X channel and Y channel output offsets, set by the above
level controls, to be switched on to either or both outputs, or to be switched off. It
therefore has four settings, as follows:-

NONE

Both X channel and Y channel output offsets are switched off.

X

The X channel output offset is switched on.

Y

The Y channel output offset is switched on.

X&Y

Both X channel and Y channel output offsets are switched on.

OUTPUT EXPAND

This control allows a ×10 output expansion to be applied to the X, Y or both output
channels, or to be switched off:-

NONE

Output expansion is turned off.

X

A ×10 output expansion is applied to the X channel output only.

Y

A ×10 output expansion is applied to the Y channel output only.

X&Y

A ×10 output expansion is applied to both the X channel and Y channel outputs.

AUTO OFFSET

This control adjusts the X offset and Y offset values so that the X channel and Y
channel outputs are zero. Any small residual values can normally be removed by
calling Auto-Offset for a second time after a suitable delay to allow the outputs to
settle.

Pressing the Previous Menu key returns control to Main Menu 1.

5.3.08 Output Equations Menu

When Main Menu 1 is displayed, pressing a key adjacent to the Output Equations
item opens the Output Equations Menu, shown in figure 5-11.

Chapter 5, FRONT PANEL OPERATION

5-16

Figure 5-11, Output Equations Menu

The Output Equations menu is used to define more complex calculations on the
instrument outputs than are possible using the basic ratio and log ratio options. There
are two user-defined equations, Equation 1 and Equation 2, which take the following
form:-













D

C B) (A

=Equation

where the operator "±" may be set to either addition or subtraction, and the variables
A, B, C and D can be chosen from the following list:-

Variable Range
X ±30000
Y ±30000
MAG 0 to +30000
PHA (Phase) ±18000
ADC1 ±10000
ADC2 ±10000
ADC3 ±10000
ADC4 ±10000
C1 0 to 100000
C2 0 to 100000
0 Zero
1 Unity
FRQ (Reference Frequency)
0 to 250000000 (Only available in position C)
OSCF(Oscillator Frequency)
0 to 250000000 (Only available in position C)

The select keys are used to highlight the required variable, and then the adjust

keys are used to change it.

The values C1 and C2 within each equation are user-defined integer constants and
are adjusted using the two corresponding keys. They may also be set using
the keypad, by pressing SET CONSTANT followed by the required value on the
keypad, or by using the Active Cursor.

Chapter 5, FRONT PANEL OPERATION

5-17

The calculation is performed using 64-bit integers to maintain full accuracy through
to the 32-bit result that is displayed immediately below the equation and is constantly
updated. Care must be taken in defining the equations so as to make the best use of
the available output range.

If the equation outputs are set to appear at any of the analog output connectors on the
rear panel using the Configuration menu, then the output range should be adjusted to
lie in the range -10000 to +10000. Values outside this range will result in these
analog outputs limiting at ±10.000 V, although the digital value will still appear
correctly on the screen and can be read via the computer interfaces.

Note that the equations continue to be calculated even when the Output Equations
menu is not displayed, if they are selected for output to the analog output connectors.
Otherwise they are calculated when requested by computer command.

Pressing the Previous Menu key returns control to Main Menu 1.

5.3.09 Oscillator Menu

When Main Menu 1 is displayed, pressing a key adjacent to the Oscillator item
accesses the Oscillator menu, which is shown in figure 5-12.

Figure 5-12, Oscillator Menu

The Oscillator menu has four controls affecting the instrument's internal oscillator,
and is also used for accessing sub-menus that control oscillator frequency and
amplitude sweeps, and frequency and amplitude modulation. The relationship of
these menus to Main Menu 1 is shown in figure 5-13. Note that as with the main
menu structure, shown in figure 5-1, it is possible to return to the Main Display from
any menu by pressing the Main Display key, but this has been omitted from figure
5-13 for the sake of clarity.

Chapter 5, FRONT PANEL OPERATION

5-18

Figure 5-13, Oscillator Menu Structure

Changes to the setting of the controls on the Oscillator menu can be made by using
the adjacent keys.

OSC FREQUENCY
This control, which duplicates the Main Display OSC FREQUENCY control, allows the
instrument's internal oscillator frequency to be set to any value between 0.001 Hz

and 250.000 kHz with a 1 mHz resolution. Adjustment is faster using the Keypad or
Active Cursor controls - see section 4.1.04.

OSC AMPLITUDE
This control, which duplicates the Main Display OSC AMPLITUDE control, sets the
amplitude of the signal at the OSC OUT connector to any value between 1 µV and

5 V rms in 1 µV increments. Adjustment is faster using the Keypad or Active Cursor
controls - see section 4.1.04.

SYNC OSCILLATOR
This control which is only active when external reference mode is active, allows the
signal at the OSC OUT connector to be derived from the drive to the X-channel
demodulator. It has two settings:

OFF

The oscillator is independent of the external reference frequency.

ON

The oscillator is a sine wave at the same frequency as the applied reference
signal but subject to any phase shift set by the phase control.

The Oscillator menu is also used to access four sub-menus, as follows:-

5.3.10 Frequency Sweep Menu

When the Oscillator menu is displayed, pressing a key adjacent to the Setup
frequency sweep item accesses the Frequency Sweep menu, which is shown in figure
5-14.

Chapter 5, FRONT PANEL OPERATION

5-19

Figure 5-14, Frequency Sweep Menu

The Frequency Sweep menu has nine controls affecting the instrument's internal
oscillator, and a link to the Curve Buffer menu (see section 5.3.24). Changes to the
setting of the controls can be made by using the adjacent keys.

When a frequency sweep is run, the internal oscillator frequency starts at the defined
start frequency and is changed in discrete steps until it reaches the stop frequency.
Steps may be of equal size, which gives a linear relationship of output frequency to
time, or may be proportional to the present frequency, which gives a logarithmic
relationship. The controls operate as follows:-

OSC FREQUENCY
This control, which duplicates the Main Display OSC FREQUENCY control, allows the
instrument's internal oscillator frequency to be set to any value between 0.001 Hz

and 250.000 kHz with a 1 mHz resolution. Adjustment is faster using the Keypad or
Active Cursor controls - see section 4.1.04.

START FREQUENCY
This control defines the start frequency for the frequency sweep, which may be set to
any value between 0.001 Hz and 250.000 kHz with a 1 mHz resolution. Adjustment
is faster using the Keypad or Active Cursor controls - see section 4.1.04.

STOP FREQUENCY
This control defines the stop frequency for the frequency sweep, which may be set to
any value between 0.001 Hz and 250.000 kHz with a 1 mHz resolution. Adjustment
is faster using the Keypad or Active Cursor controls - see section 4.1.04.

STEP SIZE

This control defines the amount by which the oscillator frequency is changed at each
step. Depending on the sweep law selected (linear or logarithmic) it is set either in
hertz, or as a percentage of the present frequency. If Start Frequency is greater than
Stop Frequency then the output frequency will decrease with time.

TIME/STEP

This control defines the time that the oscillator frequency remains at each step of the
complete frequency sweep. The range of available values is 1 ms to 1000 s in 1 ms
increments.

Chapter 5, FRONT PANEL OPERATION

5-20

Note that the time per step defined here also applies to oscillator amplitude sweeps ­see section 5.3.11.

ARMED

When this control is set to YES, the frequency sweep is armed. The sweep can then
be started in one of two ways:

a) If the LINK TO CURVE BUFFER control is set to YES then the sweep will be started

at the same time as a curve buffer acquisition starts (see section 5.3.24). This
mode allows the buffer to be used for frequency response measurements, whereby
the response of an external network or system is measured by sweeping the
oscillator frequency while recording the lock-in amplifier’s outputs, for example
magnitude and phase. Since the curve buffer can be started using internal or
external triggers there is considerable flexibility for designing experiments.

In this mode the START control is grayed out as it is inactive.
a) If the LINK TO CURVE BUFFER control is set to NO then the START control above

it is highlighted, to indicate that it is active. In this mode, the frequency sweep is
independent of the curve buffer and is operated by pressing the adjacent
key. The control annotation changes depending on whether a sweep is running, as
follows:

START

Pressing the adjacent key starts the frequency sweep. The ARMED control
changes to PAUSE

STOP

Pressing the adjacent key stops the frequency sweep.

When a sweep has been started and the ARMED control is not shown, the

following two options are also available.

PAUSE

Pressing the adjacent key pauses the frequency sweep at the present
frequency. The control changes to CONTINUE

CONTINUE

Pressing the adjacent key restarts the paused frequency sweep from the
present frequency. The control changes to PAUSE

Note that if the oscillator amplitude sweep (see section 5.3.11) is also armed then the
controls that start, pause, continue and stop on the Frequency Sweep menu will also
control the amplitude sweep. Similarly, if the frequency sweep is linked to the curve
buffer then the amplitude sweep will be as well.

LAW

This control defines the relationship of output frequency to time for the frequency
sweep, and has two options:-

LOGARITHMIC

Selects a logarithmic relationship. When in this mode, the frequency is defined
in terms of a percentage of the current frequency. For example, if the step size
were set to 10%, the start frequency to 1 kHz and the stop frequency to 2 kHz,
then the frequencies generated during the sweep would be:-

Chapter 5, FRONT PANEL OPERATION

5-21

1000.000 Hz

1610.510 Hz

1100.000 Hz

1771.561 Hz

1210.000 Hz

1948.717 Hz

1331.000 Hz

2000.000 Hz

1464.100 Hz

LINEAR

Selects a linear relationship.

Pressing the Curve Buffer key accesses the Curve Buffer menu, described in section

5.3.24, while pressing the Previous Menu key returns control to the Oscillator Menu.

5.3.11 Amplitude Sweep Menu

When the Oscillator menu is displayed, pressing a key adjacent to the Setup
amplitude sweep item accesses the Amplitude Sweep menu, which is shown in figure
5-15.

Figure 5-15, Amplitude Sweep Menu

The Amplitude Sweep menu has eight controls affecting the instrument's internal
oscillator, and a link to the Curve Buffer menu (see section 5.3.24). Changes to the
setting of the controls can be made by using the adjacent keys.

When an amplitude sweep is run, the internal oscillator output starts at the defined
start amplitude and is changed in discrete steps until it reaches the stop amplitude.
Steps are always of equal size, giving a linear relationship of output amplitude to
time. The controls operate as follows:-

OSC AMPLITUDE

This control may be set to any value between 1 µV and 5.000 V rms. Adjustment is
faster using the Keypad or Active Cursor controls - see section 4.1.04.

START AMPLITUDE

This control defines the start amplitude for the amplitude sweep, which may be set to
any value between 0.000 V rms and 5.000V rms with a 1 µV resolution. Adjustment
is faster using the Keypad or Active Cursor controls - see section 4.1.04.

Chapter 5, FRONT PANEL OPERATION

5-22

STOP AMPLITUDE

This control defines the stop amplitude for the amplitude sweep, which may be set to
any value between 0.000 V rms and 5.000V rms with a 1 µV resolution. Adjustment
is faster using the Keypad or Active Cursor controls - see section 4.1.04.

STEP AMPLITUDE

This control defines the amount by which the oscillator amplitude is changed at each
step. It may be set to any value between 0.000 V rms and 5.000V rms with a 1 µV
resolution. Adjustment is faster using the Keypad or Active Cursor controls - see
section 4.1.04.

If Start Amplitude is greater than Stop Amplitude then the oscillator amplitude will
decrease with time.

TIME/STEP

This control defines the time that the oscillator amplitude remains at each step of the
complete frequency sweep. The range of available values is 100 ms to 1000 s in 1 ms
steps.

Note that the time per step defined here also applies to oscillator amplitude sweeps ­see section 5.3.10

ARMED

When this control is set to YES, the amplitude sweep is armed. The sweep can then
be started in one of two ways:

a) If the LINK TO CURVE BUFFER control is set to YES then the sweep will be

started at the same time as a curve buffer acquisition starts (see section 5.3.24).
This mode allows the buffer to be used for linearity measurements, whereby the
response of an external network or system is measured by sweeping the oscillator
amplitude while recording the lock-in amplifier’s outputs, for example
magnitude and phase. Since the curve buffer can be started using internal or
external triggers there is considerable flexibility for designing experiments.

In this mode the START control is grayed out as it is inactive.
b) If the LINK TO CURVE BUFFER control is set to NO then the START control above

it is shown in bright text, to indicate that it is active. In this mode, the amplitude
sweep is independent of the curve buffer and is operated by pressing the adjacent

key. The control annotation changes depending on whether a sweep is

running, as follows:

START

Pressing the adjacent key starts the amplitude sweep. The ARMED control
changes to PAUSE

STOP

Pressing the adjacent key stops the amplitude sweep.

When a sweep has been started and the ARMED control is not shown, the

following two options are also available.

PAUSE

Pressing the adjacent key pauses the amplitude sweep at the present
amplitude. The control changes to CONTINUE

Chapter 5, FRONT PANEL OPERATION

5-23

CONTINUE

Pressing the adjacent key restarts the paused amplitude sweep from the
present amplitude. The control changes to PAUSE

Note that if the oscillator frequency sweep (see section 5.3.10) is also armed then the
controls that start, pause, continue and stop on the Amplitude Sweep menu will also
control the frequency sweep. Similarly, if the amplitude sweep is linked to the curve
buffer then the frequency sweep will be as well.

Pressing the Previous Menu key returns control to the Oscillator Menu.

5.3.12 Amplitude Modulation Menu

When the Oscillator menu is displayed, pressing a key adjacent to the Amplitude
modulation item accesses the Amplitude Modulation menu, which is shown in figure
5-16.

Figure 5-16, Amplitude Modulation Menu

The Amplitude Modulation menu has seven controls affecting the instrument's
internal oscillator. Changes to the setting of the controls can be made by using the
adjacent keys.

When amplitude modulation is enabled, the internal oscillator’s output amplitude can

be modulated by a signal applied to either the ADC1 auxiliary input or to the
external reference channel input (analog or TTL). The controls operate as follows:-

OSC FREQUENCY

This control may be set to any value between 0.001 Hz and 250.00 kHz. Adjustment
is faster using the Keypad or Active Cursor controls - see section 4.1.04.

MODULATION DEPTH

This control defines the maximum amplitude change that will be applied to the
oscillator signal. For example, when set to 100%, the change can be from full signal
output to zero; while if set to 50% the amplitude will change by 50% of its quiescent
level.

CENTER VOLTAGE

This control sets the input voltage corresponding to the quiescent oscillator output
amplitude.

Chapter 5, FRONT PANEL OPERATION

5-24

VOLTAGE SPAN

This control sets the range of input voltages that will be translated into the set range
of modulation.

FILTER CONTROL

The amplitude modulation control signal can be filtered by a simple digital low-pass
FIR filter before being applied to the oscillator, which can be useful for eliminating
noise or glitches on the modulating signal. This control adjusts the filter; when set to
0 the filter is turned off; as the number is increased, increased filtering is applied.

AM ENABLED

This control turns the amplitude modulation control on and off

AM SOURCE

This control determines the source of the signal used to control the oscillator
amplitude. It has two settings, as follows:

ADC 1

The control signal should be applied to the ADC1 auxiliary input. The

CENTER VOLTAGE and VOLTAGE SPAN controls are active.

EXT REF

The control signal should be applied to the external reference channel (either
TTL or analog inputs). Because the reference channel internally generates a
fixed signal amplitude that is used as the control signal for the amplitude
modulation, the CENTER VOLTAGE and VOLTAGE SPAN controls are inactive.

Pressing the Previous Menu key returns control to the Oscillator Menu.

5.3.13 Frequency Modulation (VCO) Menu

When the Oscillator menu is displayed, pressing a key adjacent to the Frequency
modulation (VCO) item accesses the Voltage Controlled Oscillator menu, which is
shown in figure 5-17.

Figure 5-17, Voltage Controlled Oscillator Menu

The Voltage Controlled Oscillator menu has seven controls affecting the instrument's
internal oscillator. Changes to the setting of the controls can be made by using the
adjacent keys.

Chapter 5, FRONT PANEL OPERATION

5-25

When the VCO is enabled, the internal oscillator’s output frequency can be

modulated by a signal applied to the ADC1 auxiliary input. The controls operate as
follows:-

CENTER FREQUENCY

This control may be set to any value between 0.001 Hz and 250.00 kHz. Adjustment
is faster using the Keypad or Active Cursor controls - see section 4.1.04.

FREQUENCY SPAN

This control defines the maximum frequency span that will be applied to the
oscillator signal. For example, when set to ±100.00 Hz, the change will be
±100.00 Hz about the quiescent level.

Note: The center frequency  the span frequency must not exceed the frequency
limits of the oscillator (0 to 250 kHz). Hence the center frequency plus the span
frequency cannot exceed 250.0 kHz and the center frequency must be greater than
or equal to the span frequency. For example, at 125 kHz center frequency the
maximum span frequency is  125 kHz; at 10k Hz center frequency the maximum
span frequency is  10 kHz; at 240 kHz center frequency the maximum span
frequency is also  10 kHz. When set from the front panel the center frequency
has priority over the span frequency.

CENTER VOLTAGE

This control sets the input voltage corresponding to the quiescent oscillator
frequency.

VOLTAGE SPAN

This control sets the range of input voltages that will be translated into the set
frequency span.

FILTER CONTROL

The VCO control signal can be filtered by a simple digital low-pass FIR filter before
being applied to the oscillator, which can be useful for eliminating noise or glitches
on the signal. This control adjusts the filter; when set to 0 the filter is turned off; as
the number is increased, increased filtering is applied.

VCO ENABLED

This control turns the VCO control on and off
Pressing the Previous Menu key returns control to the Oscillator Menu.

5.3.14 Auto Functions Menu

When Main Menu 1 is displayed, pressing a key adjacent to the Auto Functions item
accesses the Auto Functions menu, which is shown in figure 5-18

Chapter 5, FRONT PANEL OPERATION

5-26

Figure 5-18, Auto Functions Menu

This menu has five controls for activating the auto functions built into the
instrument. Note that once these functions complete, the Auto Functions menu is
replaced by the Main Display. The functions operate as follows:-

AUTO SENSITIVITY

This function only operates when the reference frequency is above 1 Hz. A single
Auto-Sensitivity operation consists of increasing the full-scale sensitivity range if the
magnitude output is greater than 90% of full-scale, or reducing the range if the
magnitude output is less than 30% of full-scale. After the Auto-Sensitivity function
is called, Auto-Sensitivity operations continue to be made until the required criterion
is met.

In the presence of noise, or a time-varying input signal, it may be a long time before
the Auto-Sensitivity sequence comes to an end, and the resulting setting may not be
necessarily what is really required.

AUTO PHASE

In an Auto-Phase operation the value of the signal phase with respect to the reference
is computed and an appropriate phase-shift is then introduced into the reference
channel so as to bring the difference between them to zero. The intended result is to
null the output of the Y channel while maximizing the output of the X channel.

Any small residual phase difference can normally be removed by calling Auto-Phase
for a second time after a suitable delay to allow the outputs to settle.

The Auto-Phase facility is normally used with a clean signal which is known to be of
stable phase. It usually gives very good results provided that the X channel and Y
channel outputs are steady when the procedure is called.

If a zero error is present on the outputs, such as may be caused by unwanted coupling
between the reference and signal channel inputs, then the following procedure should
be adopted:-

1) Remove the source of input signal, without disturbing any of the connections to
the instrument signal input which might be picking up interfering signals from
the reference channel. In an optical experiment, for example, this could be done
by shielding the detector from the source of chopped light.

Chapter 5, FRONT PANEL OPERATION

5-27

2) Execute an Auto-Offset operation, which will reduce the X channel and Y
channel outputs to zero.

3) Re-establish the source of input signal. The X channel and Y channel outputs
will now indicate the true level of input signal, at the present reference phase
setting.

4) Execute an Auto-Phase operation. This will set the reference phase-shifter to the
phase angle of the input signal. However, because the offset levels which were
applied in step 2 were calculated at the original reference phase setting, they will
not now be correct and the instrument will in general display a non-zero Y
channel output value.

5) Remove the source of input signal again.

6) Execute a second Auto-Offset operation, which will reduce the X channel and Y
channel outputs to zero at the new reference phase setting.

7) Re-establish the source of input signal.

This technique, although apparently complex, is the only way of removing the effect
of crosstalk which is not generally in the same phase as the required signal.

AUTO MEASURE

This function only operates when the reference frequency is greater than 1 Hz. It
performs the following operations:

The instrument is set to AC-coupled and input FLOAT mode. If the reference
frequency is more than 10 Hz the output time constant is set to 10 ms, otherwise it is
set to the lowest synchronous value, the filter slope is set to 12 dB/octave, output
expand is switched off, the reference harmonic mode is set to 1, the X offset and Y
offset functions are switched off and the Auto-Sensitivity and Auto-Phase functions
are called. The Auto-Sensitivity function also adjusts the AC Gain if required.

The Auto-Measure function is intended to provide a means of setting the instrument
quickly to conditions which will be approximately correct in typical simple
measurement situations. For optimum results in any given situation, it may be
convenient to start with Auto-Measure and to make subsequent modifications to
individual controls.

NOTE: The Auto-Measure function affects the setting of the AC Gain and AC
Gain Automatic controls during execution. Consequently, it may not operate
correctly if the AC Gain Automatic control is turned off. In this case, better results
will be obtained by performing Auto-Sensitivity followed by Auto-Phase functions.

AUTO OFFSET

In an Auto-Offset operation the X offset and Y offset functions are turned on and are
automatically set to the values required to give zero values at both the X channel and
Y channel outputs. Any small residual values can normally be removed by calling
Auto-Offset for a second time after a suitable delay to allow the outputs to settle.

The primary use of the Auto-Offset is to cancel out zero errors which are usually
caused by unwanted coupling or crosstalk between the signal channel and the
reference channel, either in the external connections or possibly under some
conditions in the instrument itself. Note that if a zero error is present, the Auto­Offset function should be executed before any execution of Auto-Phase.

Chapter 5, FRONT PANEL OPERATION

5-28

AUTO DEFAULT

With an instrument of the design of the model 7270, where there are many controls
of which only a few are regularly adjusted, it is very easy to overlook the setting of
one of them. Consequently an Auto-Default function is provided, which sets all the
controls to a defined state. This is most often used as a rescue operation to bring the
instrument into a known condition when it is giving unexpected results. A listing of
the settings which are invoked by the use of this function can be found in
appendix D.

Pressing the Previous Menu key returns control to Main Menu 1

5.3.15 Configuration Menu 1

When Main Menu 1 is displayed, pressing a key adjacent to the Configuration item
accesses the Configuration Menu 1, which is shown in figure 5-19.

Figure 5-19, Configuration Menu 1, Single Reference Mode

The Configuration Menu 1 has four controls used to set the instrument’s basic
operating mode, controls for the noise measurement mode, and a display of the
version of the instrument firmware. Changes to the setting of the controls can be
made by using the adjacent keys.

It is also used to access the Communications, Options, and Configuration 2 menus.
The relationship of these menus to Main Menu 1 is shown in figure 5-20. Note that
as with the main menu structure, shown in figure 5-1, it is possible to return to the
Main Display from any menu by pressing the Main Display key, but this has been
omitted from figure 5-20 for the sake of clarity.

Chapter 5, FRONT PANEL OPERATION

5-29

Figure 5-20, Configuration Menu Structure

The controls on the Configuration menu operate as follows.

NOISE MEASUREMENT

This control is used to configure the instrument for noise measurements. When
turned ON, the Main Display output displays (see section 5.3.01) are set as follows:

Display Position Displayed Output

1 Noise expressed in V/Hz in large digits
2 Noise expressed in V/Hz as a bar-graph
3 X output in expressed as a percentage of the full-scale
sensitivity as a bar-graph
4 Magnitude output (R) output in expressed as a
percentage of the full-scale sensitivity in large digits.

The available output time constant is restricted to the values in the range 500 µs to
10 ms inclusive, since it is only at these values that the noise measurements are
calibrated, and the synchronous time constant control (section 5.3.06) is turned off.
The output filter slope is also restricted to either 6 or 12 dB/octave.

When the noise measurement control is turned OFF then the instrument is configured
for normal lock-in amplifier operation.

NOISE BUFFER LENGTH

This control, which is only active when the above NOISE MEASUREMENT control is
turned ON, sets the averaging time of the buffer used to determine the mean value of
the output signal when making noise measurements. Settings of Off, 1 s, 2 s, 3 s and

4 s can set using the adjacent key.
The operation of this control is described in more detail in section 3.3.20

Firmware version X.X

This line, immediately under the menu title, gives the version number of the

Chapter 5, FRONT PANEL OPERATION

5-30

instrument's operating firmware. The firmware in the instrument can be updated to
the latest version by connecting it to a PC via the RS232 or USB interfaces and
running an Update program.

Options

This key gives access to the Options Menu, which is used to install firmware options
within the instrument. It is discussed later in section 5.3.21

Configuration 2

This key gives access to the Configuration Menu 2, discussed later in section 5.3.22

Turn display off

Pressing this key turns off the display panel. Press any key to turn it back on.
Pressing the Previous Menu key returns control to the Main Menu 1
The Virtual Reference, Dual Reference, and Dual Harmonic modes are discussed

later in sections 5.4, 5.5 and 5.6 respectively.

5.3.16 Communications Menu

When the Configuration Menu 1 is displayed, pressing the Communications key
accesses the Communications Menu, shown in figure 5-21.

Figure 5-21, Communications Menu

The Communications menu has keys to access four sub-menus. The elapsed time
indicator in the bottom right hand corner reports the total time for which the
instrument has been powered up.

Pressing the Previous Menu key returns control to the Configuration Menu 1.

5.3.17 RS232 Settings Menu

When the Communications menu is displayed, pressing the RS232 Settings key
accesses the RS232 Settings menu, which is shown in figure 5-22.

Chapter 5, FRONT PANEL OPERATION

5-31

Figure 5-22, RS232 Settings Menu

This menu has seven controls affecting the RS232 computer interface, as follows:-

BAUD RATE

This control sets the baud rate (bits per second) to one of the following values:­75 1800

110 2000

134.5 2400

150 4800
300 9600
600 19200
1200 38400

DATA BITS

This control sets the data transmission to one of four formats:­Data Bits Description

7 + 1 parity 7 data bits + 1 parity bit
8 + 1 parity 8 data bits + 1 parity bit
8 + no parity 8 data bits + 0 parity bit

ADDRESS

When more than one compatible instrument is connected in "daisy-chain" fashion by
coupling the AUX RS232 connector on the rear panel of the instrument to the RS232
port on the next, then this control is used to define the instrument's RS232 address.
All daisy-chained instruments receive commands but only the instrument currently
being addressed will implement or respond to them, except of course the command
that changes the instrument to be addressed.

ECHO

This control, when switched on, causes the model 7270 to echo each character
received over the RS232 interface back to the controlling computer. The computer
should wait until the echoed character is returned before it sends the next character.
When switched off, character echo is suppressed.

NOTE: Character echo should always be switched on, except when controlling the

Chapter 5, FRONT PANEL OPERATION

5-32

instrument manually from a simple RS232 terminal where the maximum speed
with which characters can be sent to the instrument is limited by the speed of
human typing.

PROMPT

This control has two settings, as follows:-

ON

A prompt character is generated by the model 7270 after each command
response to indicate that the instrument is ready for a new command. The prompt
character is either a "*" or a "?" If a "?" is generated, it indicates that an
overload, reference unlock, parameter error or command error has occurred.

OFF

No prompt character is generated.

DELIMITER

The character shown is that sent by the lock-in amplifier to separate two numeric
values in a two-value response, such as that generated by the MP command. The
corresponding ASCII value of this character is also shown in brackets. For example,
value 44 corresponds to a “,” (comma).

PARITY

This control sets the parity check polarity when the Data Bits control specifies that a
parity bit should be used. It should be set to match the setting of the controlling
computer.

Pressing the Previous Menu key returns control to the Communications Menu, and
pressing the Communications Monitor key accesses the Communications Monitor
display.

5.3.18 Ethernet Settings Menu

When the Communications menu is displayed, pressing a key adjacent to the
Ethernet Settings item accesses the Ethernet Settings menu, which is shown in figure
5-23.

Figure 5-23, Ethernet Settings Menu

This menu has one control, five keys to access other menus, and four indicators for

Chapter 5, FRONT PANEL OPERATION

5-33

the Ethernet computer interface, as follows:-

IP Address Assignment

This control has two settings, as follows:-

MANUAL

When set to manual, the IP address of the instrument is set manually, using the
IP Address control on this menu.

AUTOMATIC

In automatic mode, if the instrument is connected to a network with a DHCP
server then it will adopt the address given to it be the server. The address is then
shown in the IP Address field of this menu, which now functions only as an
indicator.

IP Address
Subnet Mask
Default Gateway

These three menu items behave in a similar way.
When the IP Address Assignment control is set to Manual, they allow access to

corresponding setup menus, similar to the IP Address Setup menu, shown below in
figure 5-24. Use the arrow keys or the numeric keypad to enter the required address
and then press Previous Menu to return to the Ethernet Settings menu.

When the IP Address Assignment control is set to Automatic, they function simply
as indicators showing the settings allocated by the DHCP server.

Figure 5-24, IP Address Setup Menu

Computer IP Address

In normal use, the instrument will accept connections from any IP address, and hence
can be operated from any computer on the network. In order to prevent operation
from computers other than the intended one, this control allows the address of the
controlling computer to be entered. It operates in a similar way to the other address
controls. Press a key adjacent to it to enter the Single Computer Access Setup menu,
which is similar to the IP Address Setup menu, shown in figure 5-24. Setting an IP
address of 0.0.0.0 allows access from any computer on the network; any other setting
restricts access to the computer having the specified address.

Chapter 5, FRONT PANEL OPERATION

5-34

Link Status

This indicator shows whether the instrument is connected to a working network.

Crossover Status

This indicator shows whether the cable connecting the instrument to the network is
straight-through or crossover.

Speed

This indicator shows the network speed.

Duplex

This indicator shows whether the network is using half or full duplex transmission to
and from the instrument.

Connection Statistics

Pressing a key adjacent to this menu item allows access to several menus containing
further information about the Ethernet interface, that can be useful when
troubleshooting.

Pressing the Previous Menu key returns control to the Ethernet Settings Menu.

5.3.19 USB Status Menu

When the Communications menu is displayed, pressing a key adjacent to the USB
status item accesses the USB Status menu, which is shown in figure 5-25.

Figure 5-25, USB Status Menu

This menu has five indicators reporting the USB computer interface status, one
control, and a key for accessing the Communications Monitor display, as follows:-

Manufact. Descriptor

This is the manufacturer text string as reported in the USB descriptor.

Product Descriptor

This is the product description text string as reported in the USB descriptor.

Ser. Num. Descriptor

This is the instrument serial number text string as reported in the USB descriptor.

Chapter 5, FRONT PANEL OPERATION

5-35

STATUS

This indicates whether the instrument is connected to the USB and has been
correctly enumerated by the controlling computer.

CONNECTION SPEED

If the USB status is connected, then this indicator shows the actual connection speed.

OUTPUT TERMINATOR

This has two settings, as follows:

NULL

In this setting the ASCII character responses that the instrument sends returns via
the USB interface are terminated in a null character

STATUS

In this setting the ASCII character responses that the instrument sends returns via
the USB interface are terminated with the Status and Overload bytes.

Pressing the Communications Monitor key accesses the Communications Monitor
display, while pressing the Previous Menu key returns control to the
Communications Menu.

5.3.20 Communications Monitor

When the Communications Menu is displayed, pressing the Communications
monitor key accesses the Communications monitor display, shown in figure 5-26. It
may also be accessed via the RS232 Settings and USB Status menus.

Figure 5-26, Communications Monitor

The monitor is useful when attempting to establish communications via the computer
interfaces for the first time, or if a problem is suspected.

The Input side of the display shows all of the characters that have been received from
the interface, whether valid or not. The Output side of the display shows all the
characters that have been generated by the instrument and sent to the interface.

If characters received do not match those sent by the controlling computer then this
indicates that an error has occurred either in the host computer or interface cable. If
the interface cable is known to be good, then re-check either the RS232 or USB
communications settings.

Chapter 5, FRONT PANEL OPERATION

5-36

CLEAR SCREEN

The input and output displays scroll once they are full so that they always display the
most recent characters received and sent. Pressing the Clear Screen key clears both
areas.

Pressing the Previous Menu key returns control to either the Communications menu
or the USB Status menu, or the RS232 Setting menu, depending on how the
Communications Monitor display was accessed.

Pressing the Previous Menu key returns control to the Configuration Menu.

5.3.21 Options Menu

When the Configuration menu is displayed, pressing the Options key accesses the
Options Menu, shown in figure 5-27.

Figure 5-27, Options Menu

The options menu is used to install additional firmware options and shows which
options are already fitted (note that currently there are no options available, but the
screen is provided to allow for future developments). If an option is purchased at the
same time as the instrument, then the factory will install it and no further action is
required. If however it is bought at a later date then the user will need to provide
details of the instrument's serial number with his order. In return, he will be given a
certificate with a unique eight-digit Option Key number that can be used to enable
the option on the instrument.

The controls on the Options Menu operate as follows.

INSTALL OPTION

Pressing this key displays the keypad icon. Enter the Option Key number you have
been given using the numerical keypad, and press the to enter the number. The 
symbol next to the option being installed in the list of available options on the right
hand side of the screen will change to a  to indicate successful installation.

Pressing the Previous Menu key returns control to the Configuration Menu.

5.3.22 Configuration Menu 2

When the Configuration Menu 1 is displayed, pressing the Configuration 2 key
accesses the Configuration Menu 2, shown in figure 5-28.

Chapter 5, FRONT PANEL OPERATION

5-37

Figure 5-28, Configuration Menu 2

The Configuration Menu 2 has two controls, which operate as follows:

LINE FREQUENCY

This control is used to set the center frequency of the line frequency rejection filter,
and so should be set to the local AC power line frequency, i.e. 50 or 60 Hz.

PHASE POLARITY

This control sets the measurement polarity for signal phase measurements. When set
to Positive operation is the same as earlier SIGNAL RECOVERY lock-in amplifiers,

such as the models 7220, 7225, 7265, 7265, 7280, 5209 and 5210. When set to
Negative it is the same as earlier versions of firmware in the models 7270, 7124 and

7230.

Pressing the Previous Menu key returns control to Configuration Menu 1.

5.3.23 Spectral Display

When Main Menu 1 is displayed, pressing the Spectral display key accesses the
Spectral Display menu, a typical example of which is shown in figure 5-29.

Figure 5-29, Spectral Display Menu

Chapter 5, FRONT PANEL OPERATION

5-38

The Spectral Display menu shows the result of a discrete Fourier transform (DFT) of
the signal at the input, following amplification and filtering by the line frequency
rejection and anti-aliasing filters but prior to the demodulators. Its principal function
is to allow the user to determine the relative spectral density of the total signal plus
noise being measured, so that changes can be made to the reference frequency to
avoid particularly noisy regions.

Note that the Spectral Display menu does not have a calibrated vertical axis and
hence is not intended for measuring or comparing signal amplitudes.

The central section of the display shows a plot of spectral density (vertical axis)
versus frequency (horizontal axis). The measurement frequency resolution is
adjusted by using either the Res. keys or by the or keys and can be set
to bin widths of 0.2 kHz, 0.4 kHz, 1 kHz and 2 kHz, as indicated by the control
annotation. In figure 5-24 above it is set to 0.4 kHz.

The measurement resolution sets the overall frequency range of the X-axis. At a
resolution of 2 kHz, the nominal range is 0 kHz to 458.0 kHz, although frequencies
above 250 kHz are not usually of interest since they lie outside the frequency range
of signals that the 7270 can measure. With the finest resolution of 0.2 kHz, the
display range is nominally 45.8 kHz.

When the resolution is set to 1 kHz or finer then the keys adjacent to the
Lower and Upper frequency limit indicators at each of the X-axis can be used to
scroll through the whole of the available frequency range. When either limit is
changed, the other one changes by the same amount, thereby maintaining the
frequency resolution.

LOG SCALE / LINEAR SCALE

The keys at the top right-hand corner of the display change the vertical
display between linear and logarithmic calibration, with the presently active
selection being shown.

RUN FFT

When the key adjacent to the Run FFT control is pressed, the instrument
acquires a new set of data, performs an FFT on it and displays the resulting
spectrum. A change in the reference frequency also causes the displayed spectrum to
be updated.

LIVE FFT

When the key adjacent to the Live FFT control is pressed, the instrument
acquires data repeatedly, performs an FFT on it and displays the resulting spectrum
until the Stop key is pressed.

NOTE: Live data acquisition is not available at 0.2 kHz and 0.4 kHz measurement
resolutions.

Cursor

The and cursor movement keys move the display cursor from side to side. The
cursor takes the form of a short vertical line that is positioned just above the top of
the data curve at the selected frequency, which appears just below the X-axis. In
figure 5-24 the cursor can be seen positioned at 100 kHz. The cursor makes it
possible to obtain an approximate frequency for any peak on the display, thereby
possibly assisting in identifying its source.

Chapter 5, FRONT PANEL OPERATION

5-39

Reference Frequency Indicator

The display shows the letter “R” centered above the frequency at which the lock-in’s
reference channel is currently operating. This makes it possible to quickly identify
the signal of interest from all the other signals present.

Using the Spectral Display Mode

In a typical experiment, if the lock-in amplifier’s output is more noisy than might be
expected with the given settings, then the spectrum of the total input signal being
measured can be determined and displayed using the Spectral Display menu. If
strong interfering signals close to the reference frequency are observed, then the
reference frequency should if possible be changed so that operation occurs in a
quieter region. Alternatively it may be possible to switch off the source of the
interference, which might for example be caused by a switched mode power supply
used to power another item of test equipment.

Pressing the Previous Menu key exits the Spectral Display menu and returns to
Main Menu 1.

5.3.24 Main Menu 2

When Main Menu 1 is displayed, pressing the Main menu 2 key accesses
Main Menu 2, which is shown in figure 5-30.

Figure 5-30, Main Menu 2

Main Menu 2 has keys used to access the extended features found in the model 7270,
via a series of sub-menus. The relationship of these sub-menus to Main Menu 2 is
shown in figure 5-31. Note that as with the main menu structure, shown in figure 5-1,
it is possible to return to the Main Display from the sub-menus by pressing the
Main Display key, but this has been omitted from figure 5-31 for the sake of clarity.

Chapter 5, FRONT PANEL OPERATION

5-40

Figure 5-31, Main Menu 2 Menu Structure

The Communications menu has already been described in section 5.3.16, but the
other sub menus are described in the following sections.

Pressing the Previous Menu key returns control to Main Menu 1.

5.3.25 Curve Buffer Menu

When Main Menu 2 is displayed, pressing the Curve buffer key accesses the Curve
Buffer menu, which is shown in figure 5-32.

Figure 5-32, Curve Buffer Menu

Chapter 5, FRONT PANEL OPERATION

5-41

The Curve Buffer menu has six controls affecting the instrument's internal 100,000
point curve buffer, two status indicators and keys to access the Curve Trigger and
Curve Select sub-menu, and the Oscillator menu.

CURVE MODE

The curve buffer can be used in two different modes. In Standard mode selected
instrument outputs (chosen using the Curve Select Menu) are stored in the main
instrument memory, and the fastest time per point is 1 ms. In Fast mode, up to eight
instrument outputs are stored directly in the main FPGA memory, with the fastest
time per point being reduced to 1 µs. Fast mode is a new mode in the model 7270 but
in some ways can be considered as an extension to the Transient Recorder mode
found in earlier SIGNAL RECOVERY DSP lock-in amplifiers.

TIME PER POINT

This control defines the interval between each data point in the curve buffer. In Fast
mode the range is 1 µs to 1 s in 1 µs increments; in Standard mode it is 1 ms to
1000 s in 1 ms increments

CURVE LENGTH

This control defines the number of points to be stored in the internal curve buffer. If
the Curve Mode is set to Standard, 100,000 points are available, shared equally
between the curve types as defined by the Curve Select menu. Hence, for example, if
16 curve types are to be stored then the maximum curve length for each curve is
6250 points. Note that if the number of curves to be stored is increased beyond that
which may be stored at the current curve length, then the curve length is reduced
automatically.

If the Curve Mode is set to Fast, 100,000 points are available, with either five (for
single reference modes) or eight (for dual reference/harmonic modes) being stored.

CURVE START TRIGGER

This control sets the condition used to start acquisition into the curve buffer once this
has been armed, and has three settings, as follows:-

RISING EDGE EXTERNAL

Acquisition starts on receipt of the rising edge of TTL trigger signal at the
TRIG IN connector on the rear panel of the instrument.

FALLING EDGE EXTERNAL

Acquisition starts on receipt of a falling edge of TTL trigger signal at the
TRIG IN connector on the rear panel of the instrument.

INTERNAL

Acquisition starts directly when the Start Sweep key is pressed.

CURVE STOP TRIGGER

This control sets the condition used to stop acquisition into the curve buffer once it is
running, and has four settings, as follows:-

RISING EDGE EXTERNAL

Acquisition stops on receipt of the rising edge of TTL trigger signal at the
TRIG IN connector on the rear panel of the instrument.

FALLING EDGE EXTERNAL

Acquisition stops on receipt of a falling edge of TTL trigger signal at the

Chapter 5, FRONT PANEL OPERATION

5-42

TRIG IN connector on the rear panel of the instrument.

CURVE BUFFER FULL

Acquisition stops as soon as the number of points acquired equals the Curve
Length as set on the Curve Buffer menu.

INTERNAL

Acquisition stops directly when the Stop Sweep key is pressed.

START SWEEP

Pressing this key starts data acquisition into the curve buffer.

STOP SWEEP

Pressing this key stops data acquisition into the curve buffer.

ARM SWEEP

Pressing this key arms data acquisition into the curve buffer so that is starts on
receipt of a valid trigger.

POINTS

This shows the number of points stored in the curve buffer. On completion of a
sweep when acquisition is set to stop when buffer is full (on the Curve Trigger Menu

- see section 5.3.25), the number will be the same as the Curve Length control.
(SWEEPS)

This shows the number of completed sweeps, where one sweep is equal to the
Length control setting.

Pressing the Previous Menu key returns control to the Main Menu 2.

5.3.26 Curve Trigger Menu

When the Curve Buffer menu is displayed, pressing the Curve trigger menu key
accesses the Curve Trigger menu, which is shown in figure 5-33.

Figure 5-33, Curve Trigger Menu

The Curve Trigger menu is used to control how trigger inputs affect the curve buffer,
and the position and polarity of the trigger output.

Chapter 5, FRONT PANEL OPERATION

5-43

ACQUISITION TRIGGER

The acquisition trigger is a "per curve" (start) or "per point" (sample) trigger
detecting the signal at the TRIG IN connector on the rear panel of the instrument, as
follows:

INTERNAL

The curve buffer acquisition is not affected by input triggers

EXT PER CURVE

Acquisition starts on detection of a rising edge on the TTL trigger signal. It
continues until the stop trigger condition is satisfied; each data point is taken at
the buffer time per point setting.

EXT PER POINT

Acquisition starts on detection of a rising edge on the TTL trigger signal, with
each data point requiring a new trigger. It continues until the stop trigger
condition is satisfied; the buffer time per point setting has no effect.

EXT PER CURVE

Acquisition starts on detection of a falling edge on the TTL trigger signal. It
continues until the stop trigger condition is satisfied; each data point is taken at
the buffer time per point setting.

EXT PER POINT

Acquisition starts on detection of a falling edge on the TTL trigger signal, with
each data point requiring a new trigger. It continues until the stop trigger
condition is satisfied; the buffer time per point setting has no effect.

STOP TRIGGER

This control sets the condition used to terminate acquisition into the curve buffer, as
follows:

INTERNAL

The curve buffer acquisition continues until stopped manually from the front
panel or via an HC (halt curve) computer command

CURVE BUFFER FULL

The curve buffer acquisition continues until the number of points acquired
equals the curve length setting.

EXT ON TRIG IN

The curve buffer acquisition continues until detection of a falling edge on the
TTL trigger signal.

EXT ON TRIG IN

The curve buffer acquisition continues until detection of a rising edge on the
TTL trigger signal.

TRIG OUT MODE

The instrument can generate a TTL compatible trigger output during acquisition to
the curve buffer, which is useful when synchronizing the experiment to the
acquisition. The control has two settings:-

START ONLY

A trigger output is generated once at the start of data acquisition

Chapter 5, FRONT PANEL OPERATION

5-44

SAMPLE

A trigger output is generated each time that a point is acquired into the buffer.

TRIG OUT POLARITY

This control has two settings:-

RISING

The trigger output is marked by the rising edge of the signal.

FALLING

The trigger output is marked by the falling edge of the signal.

Pressing the Previous Menu key returns control to the Curve Buffer Menu.

5.3.27 Curve Select Menu

When the Curve Buffer menu is displayed, pressing the Curve select key accesses the
Curve Select menu, which are shown in figures 5-34 and 5-35. The menus differ
depending on the setting of the curve mode control on the curve buffer menu.

Figure 5-34, Curve Select Menu - Standard Mode

Figure 5-35, Curve Select Menu - Fast Mode

For the standard mode, this menu is used to select the instrument outputs that will be

Chapter 5, FRONT PANEL OPERATION

5-45

stored when data acquisition to the curve buffer is initiated. The possible data types
(outputs) that can be stored to the curve buffer are shown on the upper part of the
screen. Controls allow between one and all of these data types to be selected for
storage, with those that are selected being indicated by being shown in highlighted
text.

MOVE POINTER

This control allows the selection box to be moved to any one of the possible data
types. The , , and cursor-movement keys can be also be used to do this.

ENTER SELECTION / CLEAR SELECTION

If the data type enclosed by a selection box is not selected, then pressing this key
causes it to be selected, as indicated by its being displayed in white text. If it is
already selected, then pressing the key deselects it.

As shown in figure 5-34, when the buffer is used in fast mode, five output values are
stored and there is no provision for changing these.

Pressing the Previous Menu key returns control to the Curve Buffer menu.

5.3.28 Single Graph Menu

When Main Menu 2 is displayed, pressing a key adjacent to the Single Graph item
accesses the Single Graph menu, which is shown in figure 5-36.

Figure 5-36, Single Graph Menu

The Single Graph menu plots the data of one curve stored in the curve buffer. This
allows real-time or post-acquisition display of selected instrument outputs in "strip
chart" format, and has a cursor which allows accurate determination of the output
value at a given sample point.

If there is no data in the curve buffer then the graph will show a straight line
representing zero. If curve storage is already running, or if there is data in the curve
buffer, then a curve will be displayed with the most recent value at the right-hand
side of the screen.

Figure 5-36 shows the layout of the Single Graph menu. The curve is displayed in
the central section of the display, with the keys on either side being used to adjust the
axes and to select the data to be shown, as follows:-

Chapter 5, FRONT PANEL OPERATION

5-46

SCALE keys
The top and bottom left-hand keys are used to adjust the upper and lower
limits of the vertical axis with a 1% resolution, the set maximum and minimum
values being shown adjacent to them.

AUTO SCALE keys

The upper and lower middle pairs of left-hand keys (or the and keys)
are used to autoscale the upper and lower limits of the vertical axis to match the
range present in the visible section of the displayed curve.

There is no facility for adjusting the x-axis scale, which always shows up to 243
points. Hence if a curve of more than this number of points is acquired it will not be
possible to show all of the points on the display at the same time.

Curve Selection keys

The keys on the top right-hand side of the display are used to select the curve to be
shown from those stored in the curve buffer. For example, if only X DATA and Y
DATA curves were specified, using the Curve Select sub-menu, as being required
then these would be the only two selections available.

START/STOP keys

In the single graph display mode, acquisition to the curve buffer, and hence display
of data, can be controlled using the Start and Stop keys.

CURSOR keys

The bottom right-hand keys and the and cursor-movement keys move
the displayed cursor, which is only active when data is not being stored, from side to
side. The present point number is shown in the bottom right-hand corner of the
screen and the value of the curve at its intersection with the cursor appears in the top
right-hand corner. Where applicable, values are given as 100 x percentage of full­scale (e.g. a value of 5000 represents 50% f.s.), since there is no facility to display
them in floating-point format. If the cursor is moved fully to the left then the
displayed data scrolls to the right in groups of ten points, allowing earlier data to be
shown.

If acquisition is in progress then the cursor is automatically positioned at the right­hand side of the display area and cannot be moved.

Pressing the Previous Menu key on the front panel exits the Single Graph menu and
returns to Main Menu 2.

5.3.29 Double Graph Menu

When Main Menu 2 is displayed, pressing a key adjacent to the Double Graph item
accesses the Double Graph menu, which is shown in figure 5-37.

Chapter 5, FRONT PANEL OPERATION

5-47

Figure 5-37, Double Graph Menu

The double graph display is similar to that of the single graph, but displays two
curves.

If there is no data in the curve buffer then the graph will show two horizontal straight
lines representing zero. If curve storage is already running, or if there is data in the
curve buffer, then two curves will be displayed with the most recent values at the
right-hand side of the screen.

Figure 5-37 shows the layout of the double graph display. The two curves are
displayed in the central section of the display, with the keys on either side being used
to adjust the axes and to select the data to be shown, as follows:-

SCALE keys

The keys to the left-hand side of the display are used to set the upper and lower
limits of the vertical axis in 1% increments, with the top and upper-middle
keys being used for the upper curve (Curve 1) and the lower-middle and bottom

keys for the lower curve (Curve 2). Pressing both sides of one of the key pairs
simultaneously automatically sets the relevant limit to match the range present in the
visible section of the displayed curve.

As in single graph mode, there is no facility for adjusting the x-axis scale, which
always shows up to 243 points. Furthermore both the upper and lower curves are
shown over the same range of points; it is not possible, for example, to show Curve 1
for data points 1 to 243 and Curve 2 for points 243 to 486.

Curve Selection keys

The keys on the top right-hand side of the display are used to select the curve to be
shown in the upper half of the display, Curve 1, from those stored in the curve
buffer. The lower-middle right-hand keys perform an equivalent function for Curve
2, which is shown in the lower half of the display. All curves that can be stored may
be selected for display, except for EVENT, frequency, and the curve recording
instrument sensitivity settings.

START/STOP keys

In the single graph display mode, acquisition to the curve buffer, and hence display
of data, can be controlled using the Start and Stop keys.

Chapter 5, FRONT PANEL OPERATION

5-48

CURSOR keys

As with the single graph mode, the bottom right-hand keys and the and
cursor-movement keys control the position of the cursor. The current point number is
displayed in the bottom right-hand corner of the display and the value of the curves
at their intersection with the cursor appear above the relevant Curve 1 and Curve 2
data types. Where applicable, values are always given as a percentage of full-scale,
since there is no facility to display them in floating-point format. If the cursor is
moved fully to the left then the displayed data scrolls to the right in groups of ten
points, allowing earlier data to be shown.

Pressing the Previous Menu key exits the Double Graph menu and returns to Main
Menu 2.

5.3.30 User Settings Menu

When Main Menu 2 is displayed, pressing a key adjacent to the User Settings item
accesses the User Settings menu, which is used to save and recall up to eight
complete instrument settings from memory. This feature is particularly useful when a
number of users share an instrument since it allows each user to quickly reset the
instrument to a known setting.

When the menu is first displayed, the Memory keys on the display are not shown.
Pressing the Save settings, Restore settings or Delete memory keys causes
Select Memory Number and the Memory keys to be displayed. For example, when
the Save settings key is pressed, the menu changes to that shown in figure 5-38.

Figure 5-38, User Settings Menu -

Save Settings

To use the user settings feature, proceed as follows:

Saving an Instrument Setting

Press the Save settings key, which will cause the Memory keys to appear.
Press either side of the key next to the Memory number in which the settings
are to be stored. A message will be displayed while the settings are saved.

Restoring an Instrument Setting

Press the Restore settings key, which will cause the Memory keys to appear.
Press either side of the key next to the Memory number from which the

Chapter 5, FRONT PANEL OPERATION

5-49

settings are to be restored. A message will be displayed while the settings are
restored.

Deleting an Instrument Setting

Press the Delete memory key, which will cause the Memory keys or those
memories containing settings information to appear. Press either side of the
key next to the Memory number at which the settings are to be deleted. A message
will be displayed while this happens.

If the menu is inadvertently activated, pressing the Previous Menu key returns
control to Main Menu 2 without saving or restoring any settings.

5.3.31 ADC Menu

When Main Menu 2 is displayed, pressing the ADC Menu key accesses the ADC
menu, which is shown in figure 5-39.

Figure 5-39, ADC Menu

The ADC menu has a control to set the auxiliary ADC inputs trigger mode, an
indicator showing the effecting time per point, four displays to show the voltages at
the instrument rear-panel ADC1 to ADC4 inputs and a key to access the Curve
Buffer menu.

The control operates as follows:

ADC TRIGGER MODE

This control has the following possible settings:-

INTERNAL

A conversion is performed on ADC1, ADC2, ADC3 and ADC4 every 1 ms, with
the results being displayed on the right of the screen and being available via the
computer interfaces.

EXTERNAL

A conversion is performed on ADC1, ADC2, ADC3 and ADC4 on receipt of a
rising edge at the TTL ADC TRIG IN connector on the rear panel on the
instrument. The maximum trigger rate is 1 kHz. The results are displayed on the
right of the screen and are available via the computer interfaces.

Chapter 5, FRONT PANEL OPERATION

5-50

BURST ADC1

The burst modes are for use in conjunction with the curve buffer operating in the
Fast mode. When Burst ADC1 is selected, a burst of conversions at the rates set
by the curve buffer TIME PER POINT control is performed on ADC1 only. The
results are stored to the curve buffer, with the number of conversions being set
by the curve length control on the Curve Buffer menu - see section 5.3.24.

BURST ADC1&2

When Burst ADC1&2 is selected, a burst of conversions at the rates set by the
curve buffer TIME PER POINT control is performed on both ADC1 and ADC2.
The results are stored to the curve buffer, with the number of conversions being
set by the curve length control on the Curve Buffer menu - see section 5.3.24.

NOTE: When either of the burst acquisition modes is selected, the instrument
automatically changes the curve buffer acquisition mode to Fast.

TIME PER POINT

This indicator shows the effective ADC sampling rate.
Pressing the Curve buffer menu item access the Curve Buffer Menu, while pressing

the Previous Menu key returns control to Main Menu 2.

5.3.32 DAC Menu

When the Main Menu 2 is displayed, pressing a key adjacent to the DAC Menu item
accesses the DAC Menu, shown in figure 5-40.

Figure 5-40, DAC Menu

The DAC menu is used to configure the signals that will appear on the four DAC
connectors on the rear panel of the instrument. On the left hand side of the panel four
controls are used to set the DC voltages of the auxiliary DAC outputs, while on the
right hand side four controls are used to select the signal that will actually be applied
to the DAC 1 to DAC 4 connectors. The controls operate as follows:

AUX DAC 1, AUX DAC 2, AUX DAC 3, and AUX DAC 4
These four controls set the voltages of the corresponding auxiliary DAC to any value
between -10.000 V and +10.000 V in 1 mV increments. Adjustment is faster using
the Keypad or Active Cursor controls - see section 4.1.04. Note that these voltages
will only appear at the connectors if the corresponding DAC SETUP on the right

Chapter 5, FRONT PANEL OPERATION

5-51

hand side of the screen is set to USER DAC.
DAC1 SETUP, DAC2 SETUP, DAC3 SETUP, and DAC4 SETUP

These four controls select which signal will be made available at the corresponding
DAC 1 to DAC 4 connectors on the rear panel of the instrument. The following
settings are available, but note that internal connection limitations mean that not all
settings are available for each DAC connector.

X% (2.5V fs)

When set to X% the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the X%fs front panel display as follows:-

X% DAC Voltage
+300 7.5 V
+100 2.5 V
0 0.0 V

-100 -2.5 V

-300 -7.5 V

Y% (2.5V fs)

When set to Y1% the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the Y%fs front panel display as follows:-

Y% DAC Voltage
+300 7.5 V
+100 2.5 V
0 0.0 V

-100 -2.5 V

-300 -7.5 V

MAG% (2.5V fs)

When set to MAG% the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the MAG%fs or R% front panel displays
as follows:-

MAG%fs DAC Voltage
+300 7.5 V
+100 2.5 V
0 0.0 V

PHASE (+9 V = +180°)

When in this setting the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the PHA or  front panel displays as
follows:-

PHA or  deg DAC Voltage
+180 9.0 V
+90 4.5 V
0 0.0 V

-90 -4.5 V

-180 -9.0 V

NOISE (2.5V fs)

When set to NOISE the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the N%fs front panel display as follows:-

Chapter 5, FRONT PANEL OPERATION

5-52

N%fs DAC Voltage
+300 7.5 V
+100 2.5 V
0 0.0 V

NOTE: When NOISE is selected as an output, the noise measurement mode

(Configuration Menu) must be ON. If it is not, a warning message is displayed
which offers the option of turning it on or deselecting NOISE as an analog
output.

RATIO (2.5V fs)

When set to RATIO the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the result of the RATIO calculation,
which is defined as follows:-

10 X output

RATIO

ADC1 Input










where X output is the X channel output as a percentage of the full-scale

sensitivity and ADC 1 is the voltage (expressed in volts) applied to the ADC 1
input connector on the rear panel of the instrument. Hence, for example, if the
instrument were measuring a 100 mV signal when set to the 500 mV sensitivity
setting, the X channel output were maximized and a 1 V signal were applied to
the ADC1 input, then the value of RATIO would be:-

0.1

10

0.5

RATIO

1.000

RATIO 2















The relationship between the voltage at the DAC connector and the RATIO

value is defined as follows:-

RATIO DAC Voltage
+7.5 7.5 V
+2.5 2.5 V
0 0.0 V

-2.5 -2.5 V

-7.5 -7.5 V

LOG RATIO

When set to LOG RATIO the corresponding DAC connector on the rear panel of
the instrument outputs a voltage related to the LOG RATIO calculation, which is
defined as follows:-

10

10 X output

LOG RATIO log

ADC1 input










where X output is the X channel output as a percentage of the full-scale

sensitivity and 1 is the voltage (expressed in volts) applied to the ADC 1 input
connector on the rear panel of the instrument. Hence, for example, if the
instrument were measuring a 100 mV signal when set to the 500 mV sensitivity