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
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