Ametek 7230 Instruction Manual

Model 7230

DSP Lock-in Amplifier

Instruction Manual

198004-A-MNL-C

Copyright © 2017 AMETEK ADVANCED MEASUREMENT TECHNOLOGY, INC

Firmware Version

The instructions in this manual apply to operation of a Model 7230 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 can 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

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

Chapter One, Introduction

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

1.2 What is a Lock-in Amplifier? ......................................................................................................................... 1-10

1.3 Key Specifications and Benefits .................................................................................................................... 1-16

Chapter Two, Installation and Initial Checks

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

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

2.1.02 Rack Mounting ........................................................................................................................................ 2-3

2.1.03 Inspection ................................................................................................................................................ 2-4

2.1.04 Line Cord Plug ........................................................................................................................................ 2-5

2.1.05 Line Voltage Selection ............................................................................................................................ 2-6

2.2 Initial Checks .................................................................................................................................................... 2-7

2.2.01 Introduction ............................................................................................................................................. 2-7

2.2.02 Procedure ............................................................................................................................................... 2-11

Chapter Three, Technical Description

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

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

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

3.2.02 Single Reference / Dual Reference ......................................................................................................... 3-4

3.2.03 Tandem Demodulation ............................................................................................................................ 3-7

3.2.04 Single Harmonic / Dual Harmonic .......................................................................................................... 3-9

3.2.05 Internal / External Reference Mode ...................................................................................................... 3-11

3.2.06 Virtual Reference Mode ........................................................................................................................ 3-12

3.3 Principles of Operation ................................................................................................................................... 3-16

3.3.01 Block Diagram ....................................................................................................................................... 3-16

3.3.02 Signal Channel Inputs............................................................................................................................ 3-20

3.3.03 Line Frequency Rejection Filter ............................................................................................................ 3-29

3.3.04 AC Gain and Dynamic Reserve............................................................................................................. 3-30

3.3.05 Anti-Aliasing Filter ............................................................................................................................... 3-42

3.3.06 Main Analog-to-Digital Converter ........................................................................................................ 3-50

3.3.07 Reference Channel Inputs ..................................................................................................................... 3-53

3.3.08 Reference Channel ................................................................................................................................ 3-55

3.3.09 Phase-Shifter ......................................................................................................................................... 3-62

3.3.10 Internal Oscillator - General .................................................................................................................. 3-69

3.3.11 Internal Oscillator - Update Rate ........................................................................................................... 3-70

3.3.12 Internal Oscillator - Frequency & Amplitude Sweeps .......................................................................... 3-71

3.3.13 Internal Oscillator - Voltage Control..................................................................................................... 3-77

3.3.14 Demodulators - Dual Phase Multipliers ................................................................................................ 3-78

3.3.15 Demodulators - Output Filters ............................................................................................................... 3-80

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TABLE OF CONTENTS

3.3.16 Fast Curve Buffer ...................................................................................................................................3-87

3.3.17 Main Output Processor - General ...........................................................................................................3-91

3.3.18 Main Output Processor - Output Offset and Expand .............................................................................3-93

3.3.19 Main Output Processor - Vector Magnitude and Phase .........................................................................3-94

3.3.20 Main Output Processor - Noise Measurements ....................................................................................3-103

3.3.21 Main Output Processor - Standard Curve Buffer .................................................................................3-112

3.3.22 Analog Outputs (DACs) .......................................................................................................................3-114

3.3.23 Auxiliary Analog Inputs (ADCs) .........................................................................................................3-119

3.3.24 Main Microprocessor - General ...........................................................................................................3-121

3.3.25 Main Microprocessor - Auto Functions ...............................................................................................3-123

3.4 General ..........................................................................................................................................................3-135

3.4.01 Accuracy ...............................................................................................................................................3-136

3.4.02 Power-up Defaults ................................................................................................................................3-137

Chapter Four, Front and Rear Panels

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

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

4.1.02 REF IN Connector ....................................................................................................................................4-5

4.1.03 OSC OUT Connector ...............................................................................................................................4-6

4.1.04 STATUS Indicator ...................................................................................................................................4-7

4.2 Rear Panel .......................................................................................................................................................4-17

4.2.01 Power Input Connector ...........................................................................................................................4-20

4.2.02 Power Switch..........................................................................................................................................4-21

4.2.03 DIGITAL I/O Connector ........................................................................................................................4-22

4.2.04 RS232 Connector ...................................................................................................................................4-23

4.2.05 LAN Connector ......................................................................................................................................4-25

4.2.06 USB Connector ......................................................................................................................................4-26

4.2.07 CONFIG Switches ..................................................................................................................................4-27

4.2.08 SIG MON Connector..............................................................................................................................4-33

4.2.09 TRIG IN Connector ................................................................................................................................4-34

4.2.10 TRIG OUT Connector ............................................................................................................................4-35

4.2.11 ADC 1, ADC 2, Auto Measure ADC 3, and ADC 4 Connectors ..........................................................4-36

4.2.12 DAC 1, DAC 2, DAC 3, and DAC 4 Connectors ..................................................................................4-37

Chapter Five, Web Control Panel Operation

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

5.2 Ethernet Connection Methods ...........................................................................................................................5-3

5.2.01 Direct Wired Connection to a Single Computer ......................................................................................5-4

5.2.02 Wireless Connection to an iPad, Tablet, Laptop, or Netbook Computer .................................................5-6

5.2.03 Wired Connection to a Company or Corporate Network Using a Static IP Address .............................5-29

5.2.04 Wired Connection to a Company or Corporate Network Using a DHCP Allocated IP Address...........5-46

5.3 Web Control Panels .........................................................................................................................................5-63

5.3.01 Main Controls: Overview .......................................................................................................................5-63

5.3.02 Main Controls: Display Indicators .........................................................................................................5-79

5.3.03 Main Controls: Input ..............................................................................................................................5-87

5.3.04 Main Controls: Reference 1 .................................................................................................................5-106

5.3.05 Main Controls: Oscillator .....................................................................................................................5-123

5.3.06 Main Controls: Output 1 ......................................................................................................................5-127

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TABLE OF CONTENTS

5.3.07 Main Controls: Reference 2 ................................................................................................................ 5-142

5.3.08 Main Controls: Output 2 ..................................................................................................................... 5-153

5.3.09 Main Controls: Output Filters ............................................................................................................. 5-163

5.3.10 Main Controls: Demodulator Control ................................................................................................. 5-167

5.3.11 Main Controls: Status Indicators ......................................................................................................... 5-176

5.3.12 Oscillator: Overview ........................................................................................................................... 5-183

5.3.13 Oscillator: Sweep Control ................................................................................................................... 5-186

5.3.14 Oscillator: Modulation Control ........................................................................................................... 5-199

5.3.15 Oscillator: Amplitude and Frequency Controls .................................................................................. 5-213

5.3.16 Rear Panel: Overview .......................................................................................................................... 5-216

5.3.17 Rear Panel: DACs ............................................................................................................................... 5-219

5.3.18 Rear Panel: ADCs ............................................................................................................................... 5-249

5.3.19 Rear Panel: RS232 ............................................................................................................................... 5-253

5.3.20 Rear Panel: Ref Mon ........................................................................................................................... 5-261

5.3.21 Rear Panel: Expand ............................................................................................................................. 5-264

5.3.22 Rear Panel: Bind .................................................................................................................................. 5-268

5.3.23 Rear Panel: Digital Port ....................................................................................................................... 5-271

5.3.24 Rear Panel: DIP switches .................................................................................................................... 5-275

5.3.25 Rear Panel: USB Status ....................................................................................................................... 5-283

5.3.26 Equations: Overview ........................................................................................................................... 5-286

5.3.27 Equations: Equation 1 and Equation 2 ................................................................................................ 5-289

5.3.28 Equations: Command Interface ........................................................................................................... 5-301

5.3.29 Equations: Auto Default ...................................................................................................................... 5-304

5.3.30 Equations: Auto Measure .................................................................................................................... 5-306

Chapter Six, Computer Operation

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

6.2 Capabilities ....................................................................................................................................................... 6-3

6.2.01 General .................................................................................................................................................... 6-3

6.2.02 Operation ................................................................................................................................................. 6-4

6.2.03 Compound Commands ............................................................................................................................ 6-6

6.3 RS232 Operation .............................................................................................................................................. 6-7

6.3.01 Introduction ............................................................................................................................................. 6-7

6.3.02 General Features ...................................................................................................................................... 6-8

6.3.03 Choice of Baud Rate .............................................................................................................................. 6-14

6.3.04 Choice of Number of Data Bits ............................................................................................................. 6-16

6.3.05 Choice of Parity Check Option ............................................................................................................. 6-17

6.3.06 Handshaking and Echoes ....................................................................................................................... 6-19

6.3.07 Terminators ........................................................................................................................................... 6-25

6.3.08 Delimiters .............................................................................................................................................. 6-27

6.3.09 Status Byte, Prompts and Overload Byte .............................................................................................. 6-29

6.4 USB Operation ............................................................................................................................................... 6-37

6.4.01 Introduction ........................................................................................................................................... 6-37

6.4.02 General Features .................................................................................................................................... 6-40

6.4.03 Terminator, Status Byte, and Overload Byte ........................................................................................ 6-41

6.4.04 Delimiters .............................................................................................................................................. 6-48

6.5 Ethernet Operation ......................................................................................................................................... 6-49

6.5.01 Introduction ........................................................................................................................................... 6-49

6.5.02 IP Address ............................................................................................................................................. 6-50

6.5.03 Main Controls ........................................................................................................................................ 6-51

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TABLE OF CONTENTS

6.5.04 Sockets ...................................................................................................................................................6-52

6.5.05 Terminator, Status Byte, and Overload Byte .........................................................................................6-54

6.5.06 Delimiters ...............................................................................................................................................6-61

6.7 Command Descriptions ...................................................................................................................................6-70

6.7.01 Signal Channel .......................................................................................................................................6-71

6.7.02 Reference Channel .................................................................................................................................6-93

6.7.03 Signal Channel Output Filters ..............................................................................................................6-112

6.7.04 Signal Channel Output Amplifiers .......................................................................................................6-125

6.7.05 Instrument Outputs ...............................................................................................................................6-132

6.7.06 Internal Oscillator .................................................................................................................................6-157

6.7.07 Analog Outputs ....................................................................................................................................6-187

6.7.08 Digital I/O ............................................................................................................................................6-198

6.7.09 Auxiliary Inputs ....................................................................................................................................6-204

6.7.10 Output Data Curve Buffer ....................................................................................................................6-207

6.7.11 Computer Interfaces .............................................................................................................................6-266

6.7.12 Instrument Identification ......................................................................................................................6-287

6.7.13 Auto Default and Calibration ...............................................................................................................6-291

6.7.14 Dual Mode Commands ........................................................................................................................6-295

6.5 Programming Examples ................................................................................................................................6-304

6.5.01 Introduction ..........................................................................................................................................6-304

6.5.02 Basic Signal Recovery .........................................................................................................................6-305

6.5.03 Frequency Response Measurement ......................................................................................................6-310

6.5.04 X and Y Output Curve Storage Measurement .....................................................................................6-315

Appendix A, Specifications

Appendix B, Pinouts

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

B2 Digital I/O Port Connector ............................................................................................................................... B-1

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

iv

TABLE OF CONTENTS

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.

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:

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.

v

TABLE OF CONTENTS

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 7230 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.

vi

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 7230 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:

TABLE OF CONTENTS

BS EN61326-1:2006 Electrical equipment for measurement control and laboratory use -

EMC requirements – Class A.

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: November 2010

vii

Introduction

1.1 How to Use This Manual

This manual gives detailed instructions for setting up and operating the
SIGNAL RECOVERY Model 7230 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 7230.

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 and indicator as referred to in the subsequent

chapters.

Chapter 1

Chapter 5 - Web Control Panel Operation

Describes the capabilities of the instrument when operated via the built-in web
control panels.

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

1-1

Chapter 1, INTRODUCTION

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. More recently 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 7230 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.

1-2

1.3 Key Specifications and Benefits

The SIGNAL RECOVERY Model 7230 represents a further significant advance in
the application of DSP technology in the design of a lock-in amplifier.

Key specifications include:
 Frequency range:

Standard unit 0.001 Hz to 120.000 kHz
With 7230/99 option 0.001 Hz to 250.000 kHz

 Voltage sensitivity: 10 nV to 1 V full-scale
 Current input mode sensitivities: 10 fA to 1 µA full-scale

10 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

Chapter 1, INTRODUCTION

 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 otherwise require two lock-in amplifiers

 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
 Standard USB, Ethernet, and RS232 interfaces
 100,000 point internal curve storage buffer

1-3

Chapter 1, INTRODUCTION

1-4

Installation and
Initial Checks

2.1 Installation

2.1.01 Introduction

Installation of the model 7230 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 7230 does not use forced-air ventilation; however it should be located so
that there is reasonable flow of air around it to aid cooling.

2.1.02 Rack Mounting

An optional accessory kit, part number K02006, is available from
SIGNAL RECOVERY to allow the model 7230 to be mounted in a standard 19-inch

rack.

Chapter 2

2.1.03 Inspection

Upon receipt the model 7230 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 7230 is powered from the model PS0110 remote power module that in
turn is fitted with a standard IEC 320 input socket. A suitable line power cord is
supplied.

2.1.05 Line Voltage Selection

The model PS0110 is suitable for line voltages in the range 100 - 240 V AC, 47 ­63 Hz, and no adjustment is needed to accommodate this range. It is internally
protected against short circuit and overload and in the event of failure cannot be
repaired and must be replaced.

2.2 Initial Checks

2.2.01 Introduction

The following procedure checks the performance of the model 7230. 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

2-1

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-2

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.

The procedure requires the use of a computer with an Ethernet 10 or 100 Base T
adaptor with RJ45 connector set to support TCP/IP protocol with an installed web
browser. As an example, a Windows 7 PC with Internet Explorer 11 is suitable, but
so are many other computer systems.

2.2.02 Procedure

1) Check that the rear panel Config 1 and Config 2 switches are set to 0. This will

set the model 7230 to use the default static IP address of 169.254.150.230

2) Close all open programs on the computer and unplug any existing network
connection. Disable any wireless connection as well.

3) Plug one end of the supplied RJ45 patch cord to the computer and the other end
into the LAN connector on the rear panel of the model 7230

5) With the rear-panel mounted power switch set to 0 (off), plug the line cord into
the model PS0110 power supply unit and the 5-pin DIN plug on the power cable
from the PS0110 to the 7230’s rear panel POWER INPUT connector.

6) Turn on the model 7230. The front panel status light should be green.

7) Open a browser session on the computer. Since there is no connection to the
internet you will not see the normal opening page, but an error message. If using
Internet Explorer on Windows 10 the message will be as shown in figure 2-2;
other browsers will generate similar messages.

8) Type 169.254.150.230 into the address bar and press <return>. The 7230's Main
Controls panel should be displayed, as shown below in figure 2-3.

Figure 2-2, Initial Browser Window

Chapter 2, INSTALLATION AND INITIAL CHECKS

Figure 2-3, Model 7230 Main Controls Panel

9) Click the Equations tab, to show the Equations panel, as shown below in figure
2-4

Figure 2-4, Model 7230 Equations Panel

10) Click the Auto Default button. This will set all instrument controls to the factory

11) Connect a BNC cable from the front panel OSC OUT to the A input connector

default values,

on the front panel. The X1 and Magnitude outputs should read 100 ± 1%, and the
Y1 and Phase 1 outputs should read close to zero, as shown in figure 2-5 below.

2-3

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-4

Figure 2-5, Model 7230 Main Controls Panel – Default Settings

12) Save the address as a favorite, bookmark, or shortcut (figure 2-6) so that you can
quickly reach the model 7230 again if repeating the test, as in figure 2-7

Figure 2-6, Saving the Model 7230’s IP address as a favorite

Chapter 2, INSTALLATION AND INITIAL CHECKS

Figure 2-7, Accessing the favorite

13) 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.

2-5

Chapter 2, INSTALLATION AND INITIAL CHECKS

2-6

Technical Description

3.1 Introduction

The model 7230 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 7230 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 7230
this is referred to as the single reference mode.

Chapter 3

The dual reference mode incorporated in the model 7230 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 3.0 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 first set of demodulators operates at the carrier frequency. 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.

3.2.04 Single Harmonic / Dual Harmonic

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

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Chapter 3, TECHNICAL DESCRIPTION

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 7230 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 the upper frequency limit of the instrument, normally 120 kHz, but 250 kHz
in instruments fitted with the 7230/99 option.

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

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 the lock-in
amplifier's external reference input. The instrument's reference channel "locks" to
this signal and uses it to measure the applied input signal.

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 7230 is a very 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.

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Chapter 3, TECHNICAL DESCRIPTION

Figure 3-1, Model 7230 - 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

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Chapter 3, TECHNICAL DESCRIPTION

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.

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.

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Chapter 3, TECHNICAL DESCRIPTION

Input Limit (pk)

DR 0.7

Full-Scale Sensitivity (rms)



AC Gain (dB) INPUT LIMIT (zero to peak)
0 2.5 V
6 1.2 V
12 625 mV
18 312 mV
24 156 mV
30 78 mV
36 39 mV
42 19 mV
48 10 mV
54 5.0 mV

60 2.5 mV

66 1.2 mV
72 625 µV
78 312 µV
84 156 µV
90 78 µV

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

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). The dynamic
reserve is often expressed in decibels, for which

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Chapter 3, TECHNICAL DESCRIPTION

))ratio a log(DR(as20dB)DR(in 

Applying this formula to the model 7230 at the maximum value of INPUT LIMIT
(2.5 V) and the smallest available value of FULL-SCALE SENSITIVITY (10 nV),
gives a maximum available dynamic reserve of about 165 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.

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

f

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

sampling

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

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

signal

frequency must be uniquely generated by the signal to be

signal

= |f1 - nf

alias

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

sampling

alias

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|). If 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 400 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
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

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Chapter 3, TECHNICAL DESCRIPTION

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 for
direct user use by downloading the data to a computer.

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

External reference signals are normally applied to the model 7230 via the front panel
REF IN connector. Internally this can be switched to function as a general-purpose
input, designed to accept virtually any periodic waveform with a 50:50 mark-space
ratio and of suitable amplitude, or specifically set to accept TTL-logic level signals.
Following the trigger buffering circuitry the selected reference signal is routed to the
reference channel.

In dual reference mode where two external reference inputs can be used, one
reference is applied to the front panel REF IN connector, while the second, which
must be of 3.0 kHz or lower frequency, is applied to the rear-panel TRIG IN
connector.

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 can each be derived from the internal
oscillator, the front panel REF IN reference input, or the rear panel TRIG IN input.
The reference circuit generates new phase values for each individual channel and
sends these to the 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

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Chapter 3, TECHNICAL DESCRIPTION

external references.

External Reference Mode

In external reference mode the reference is taken from the front panel external
reference input, except in dual external reference mode, when the second reference is
applied to the rear panel TRIG IN input.

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 most 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, it is possible to configure the rear panel TRIG OUT connector to output
a TTL logic signal at the present reference frequency.

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 to 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 general-purpose setting of the external reference channel input detects positive­going 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 when the channel is configured for TTL-logic level signals
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
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

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Chapter 3, TECHNICAL DESCRIPTION

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 7230, in common with many other lock-in amplifiers, incorporates an
internal oscillator, which may be used to drive the experiment. However, unlike
older instruments, the oscillator in the model 7230 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

120.0 or 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 update rate 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
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. This cannot, of course, be used as a
source for the measurement.

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Chapter 3, TECHNICAL DESCRIPTION

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 7230 to apply to the envelope of the frequency response function of the
digital finite impulse response (FIR) output filters. Accordingly the web control
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
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.

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Chapter 3, TECHNICAL DESCRIPTION

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 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.

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

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Chapter 3, TECHNICAL DESCRIPTION

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
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.)

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Chapter 3, TECHNICAL DESCRIPTION

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

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.

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 Main Controls web panel 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 read via the computer interfaces while the time constant or slope are
outside the permitted range, or if the synchronous time constant 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 Main Controls web panel or by a
computer command. When this is turned on, the 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

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Chapter 3, TECHNICAL DESCRIPTION

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 which 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 7230 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 Rear Panel web
control panel or via a command sent over one of the interfaces. 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.

3.3.23 Auxiliary Analog Inputs (ADCs)

The model 7230 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).

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Chapter 3, TECHNICAL DESCRIPTION

3.3.24 Main Microprocessor - General

All functions of the instrument are under the control of a microprocessor, which in
addition supports the computer interfaces. It also controls 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 - 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.

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

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Chapter 3, TECHNICAL DESCRIPTION

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.

3.4 General

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.

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

When the rear panel CONFIG Switch 1 is set to 1, 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 curve buffer is cleared.
b) Any sweep that was in progress at switch-off is terminated.

3-16

Front and Rear Panels

4.1 Front Panel

Figure 4-1, Model 7230 Front Panel Layout

As shown in figure 4-1, the model 7230's front panel has four BNC connectors and
an LED Status indicator. 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.

Chapter 4

4.1.02 REF IN Connector

This is the general-purpose input connector for the external reference signal. It will
accept either analog waveforms (e.g. sinusoidal or pulse), or TTL logic signals,
depending on the setting of the reference input control.

NOTE: If the best possible phase accuracy at low external reference frequencies is
required, then a TTL reference signal should be used.

When using dual external reference modes, one of the external references is applied
to this connector and the other to the rear-panel TRIG IN connector (see section

4.2.09).

4.1.03 OSC OUT Connector

This is the output connector for the internal oscillator.

4.1.04 STATUS Indicator

This bicolor LED indicates various operating conditions within the instrument, as
follows:

Instrument Turned Off

The Status indicator is unlit

Instrument Turned On and Operating Normally

The Status indicator is permanently green

Instrument Turned On and Displaying Ethernet IP Address

When the model 7230 is turned on and has determined a valid IP address, either from
a DHCP server, or the Auto IP address function, or is using a static address, then the
Status indicator flashes as it “counts out” the actual IP address. If these flashes are
counted then it is possible to determine the address, as follows.

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Chapter 4, FRONT AND REAR PANELS

An IP address consists of four numbers, each known as an octet and each of which
can range from 0 t0 255. The model 7230 displays these numbers by using red and
green flashes of the Status indicator, as follows. A long red flash indicates the start
and end of the address, short red flashes are digit separators, and the number of green
flashes equals the digit value, except that 10 green flashes equates to a value of 0

Step No. Status Color Description
1 Long Red Start of 1st octet
2 Green Flashes Octet 1, digit 1
3 Short Red End of 1st digit, start of 2nd
4 Green Flashes Octet 1, digit 2
5 Short Red End of 2nd digit, start of 3rd
6 Green Flashes Octet 1, digit 3
7 Short Red End of 1st octet, start of 2nd
8 Green Flashes Octet 2, digit 1
9 Short Red End of 1st digit, start of 2nd
10 Green Flashes Octet 2, digit 2
11 Short Red End of 2nd digit, start of 3rd
12 Green Flashes Octet 2, digit 3
13 Short Red End of 2nd octet, start of 3rd
14 Green Flashes Octet 3, digit 1
15 Short Red End of 1st digit, start of 2nd
16 Green Flashes Octet 3, digit 2
17 Short Red End of 2nd digit, start of 3rd
18 Green Flashes Octet 3, digit 3
19 Short Red End of 3rd octet, start of 4th
20 Green Flashes Octet 4, digit 1
21 Short Red End of 1st digit, start of 2nd
22 Green Flashes Octet 4, digit 2
23 Short Red End of 2nd digit, start of 3rd
24 Green Flashes Octet 4, digit 3
25 Long Red End of 4th octet and address

If one loses track of the number of flashes, or miscounts them, then the display
process can be restarted by changing the setting of the rear panel Config 3 switch, as

described in section 4.2.07 It can also be restarted by cycling the instrument’s power,

but if using an auto IP allocated address, or if the DHCP server takes time to allocate
an address, then it cannot begin until the new IP address has been determined, so it
will take longer.

4-2

4.2 Rear Panel

4.2.01 Power Input Connector

Chapter 4, FRONT AND REAR PANELS

Figure 4-5, Model 7230 Rear Panel Layout

As shown in figure 4-5, the power input socket, power switch, digital I/O port,
RS232, USB, LAN connectors, and eleven BNC signal connectors are mounted on
the rear panel of the instrument. Brief descriptions of these are given in the following
text.

The model 7230 requires regulated DC power supply voltages of 15 V and +5 V to
operate, which are provided via the SIGNAL RECOVERY Model PS0110 power

supply module that plugs into this 5-pin DIN power input connector.

CAUTION: The Model 7230 must only be powered from the Model PS0110 power
supply module provided with it. Use of other power supplies, except with the prior
written permission of SIGNAL RECOVERY, will invalidate the warranty.

4.2.02 Power Switch

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.

4.2.03 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, 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 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.

4.2.05 LAN Connector

This connector allows the instrument to be connected to a 100-BaseT or 10-BaseT
network for control from a computer connected to the same network using either the
built-in web based control panels or separate software packages, such as a LabVIEW
driver, SIGNAL RECOVERY Acquire, or SRInstComms. The IP address can be set

4-3

Chapter 4, FRONT AND REAR PANELS

to a static value, or to use an automatic IP address assignment, or to accept an
address allocated by a DHCP server on the network. The position of CONFIG
Switch 2 (see section 4.2.07) controls this behavior.

4.2.06 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
both Full Speed and Hi-Speed (USB 2.0) protocols.

4.2.07 CONFIG Switches

These three switches are used to set the instrument’s configuration, as follows:

Switch 1: Retain Settings

This switch determines whether the instrument will retain its settings during power­down, according to the following table:

Position Description
0 Unit powers on with all settings set to factory default values (see
Appendix D)
1 Unit powers on with the same settings as when it was powered off

Hence to perform a reset to the factory default settings, turn off the power switch, set
Switch 1 to position 0, and turn the power switch back on. Switch 1 can then be set
back to 1 if desired.

Switch 2: IP Address Control

This switch selects the mode of operation of the Ethernet interface, according to the
following table:

Position Description
0 The model 7230 uses a static IP address, as set by the command

IPADDR. This command can be sent via any one of the three interfaces,
but if used via the Ethernet interface and specifying a different address to
that currently in use, then communications will be lost.

1 Unit uses a DHCP or AutoIP allocated address. It initially tries to get an

IP address via a DHCP server but if one is not assigned then it will auto
assign an IP address and test that it does not conflict with anything else
on the network. The AutoIP address will be in the range

169.254.XXX.XXX

Switch 3: IP Address Display

When this is switch is changed from 0 to 1 or from 1 to 0 then the current IP address
(static, DHCP, or Auto IP allocated) will be shown by means of flashing the front
panel STATUS light red and green.

4.2.08 SIG MON Connector

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

4.2.09 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), and for triggering acquisition into the data buffer. The input operates on the

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Chapter 4, FRONT AND REAR PANELS

positive edge only.

In dual reference mode where “Rear Panel” is selected as one of the external

reference inputs, a TTL reference signal (3.0 kHz or lower) should be applied to this
connector. Naturally this precludes using the same input for ADC triggering at the
same time.

4.2.10 TRIG OUT Connector

This connector can be set to generate a TTL-compatible output pulse, for example to
indicate when data is being sampled when using the internal curve buffer. It can also
be switched to perform an alternative function as a Reference Monitor, in which case
it outputs a TTL signal at the same frequency as the main reference channel.

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

The input voltages at these connectors are sampled and held when the ADC is
triggered, and are then digitized. Several different trigger modes are available. The
input voltage range is ±10.000 V and the resolution is 1 mV.

4.2.12 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 general­purpose outputs, the output range is ±10.000 V

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Chapter 4, FRONT AND REAR PANELS

4-6

Web Control Panel Operation

5.1 Introduction

This chapter describes how to operate the model 7230 using the built in web control
panels via the Ethernet interface, and discusses its capabilities when used in this
way. Chapter 6 provides similar information for when the unit is operated remotely
by commands sent via the Ethernet, USB, or serial interfaces.

In order to use the built in web control panels, the instrument must be connected to
an Ethernet network, it must be possible to access this same network from the
computer that will be used to operate it, and the instrument’s IP address must be
known. The following sections describe four different configurations that allow this;
use whichever is appropriate to your requirements.

5.2 Ethernet Connection Methods

5.2.01 Direct Wired Connection to a Single Computer

This is the method described in section 2.2.02 in which the model 7230 is operated
using its default static IP address of 169.254.150.230 It has the advantage of
simplicity but the disadvantage that the controlling computer does not have access to
other network resources or the internet.

Chapter 5

With this configuration, it is possible to add internet access to the computer by
adding a second Ethernet interface, either internal or via a USB-Ethernet adaptor,
and connecting this second port to an Ethernet network that has internet access.
However the model 7230 will still only be able to be controlled from the original
computer.

5.2.02 Wireless Connection to an iPad, Tablet, Laptop, or
Netbook Computer

To operate the instrument from a wireless device, such as an iPad, you will need a
wireless router that includes a DHCP server. These are widely available from
electronics stores and there are many different suppliers. The following procedure
uses a TP-Link Model TL-MR3020, but the same principles can be applied to using
routers from other manufacturers. Proceed as follows:

1) Unpack the wireless router and follow the instructions supplied with it to

connect it to the local line power.

2) Set the switch on the side of it to WISP (Wireless Internet Service Provider)

3) Turn the router on. After a short delay its wireless network will be activated.

4) Using the wireless device that you want to use to control the model 7230, such as

an iPad, laptop, or smartphone, find the wireless network that you have just
created. For the TP-Link Model TL-MR3020 the default wireless network name
is TP-LINK_POCKET_3020_XXXXXX (where XXXXXX is the last six
characters of the router’s MAC address), as shown in figure 5-1 below

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Chapter 5, WEB CONTROL PANEL OPERATION

Figure 5-1, Connecting to a Wireless Network

5) Connect to this network. In the example shown in figure 5-1, click the TP­LINK_POCKET_3020_ABE9A2.

6) Enter the wireless network security information, if required. In the case of the
TP-Link Model TL-MR3020 the wireless security key is printed on a label fixed
to the unit.

7) You now need to access the router’s web control panel, for which you will need
to know its IP address, which is typically at the beginning or end of one of the
private IP address ranges, such as 192.168.0.1 or 192.168.0.254, or 10.0.0.1 or

10.0.0.254. Consult the documentation that came with the router to find out
more; for the TP-Link Model TL-MR3020 the default address is 192.168.0.254

8) Open a browser session and type the router’s IP address into the address bar.
You may then be prompted for a username and password as in figure 5-2, which
in the case of the TP-Link Model TL-MR3020 are simply both “admin” by
default.

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Chapter 5, WEB CONTROL PANEL OPERATION

Figure 5-2, Router Home Page Login Screen

9) Enter the user name and password to access the configuration page; the panel for

the TP-Link Model TL-MR3020 is shown in figure 5-3 below.

10) Locate the DHCP information/controls page; for the TP-Link Model TL-

Figure 5-3, Router Home Page

MR3020 this is done by clicking the DHCP link and then the DHCP Clients List.
The page is shown in figure 5-4.

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Chapter 5, WEB CONTROL PANEL OPERATION

Figure 5-4, Router DHCP Status Page

11) Allow a few minutes for the router to be updated. After this, the DHCP server
page should show a single IP address – this will be address of the computer from
which you are accessing the router. In the above example the controlling
computer’s IP address is 192.168.0.100, and the router’s address (as shown, in
the address bar) is 192.168.0.254

12) Take the RJ45 patch cord and plug one end into the router and other into the
LAN connector of the rear panel of the model 7230.

13) Set the Config 2 switch on the rear panel of the model 7230 to position 1. This
will cause the instrument to be allocated an IP address by the DHCP server in the
router.

14) With the rear-panel mounted power switch set to 0 (off), plug the line cord into
the model PS0110 power supply unit and the 5-pin DIN plug on the power cable
from the PS0110 to the 7230’s rear panel POWER INPUT connector.

15) Turn on the model 7230. The front panel status light should initially be green,

but will then flash red and green as it “counts out” the IP address it has been

allocated by the router. It is not necessary to wait until this finishes before
continuing.

16) Refresh the router’s control DHCP control panel. The newly connected model
7230 will now appear as a second entry in the table:

5-4

Chapter 5, WEB CONTROL PANEL OPERATION

Figure 5-5, Router DHCP Status Page with Model 7230 Active

In the above example the controlling computer’s IP address is 192.168.0.100 and

the model 7230 has been allocated an IP address of 192.168.0.101

15) Open a new browser session and type the IP address for the model 7230
determined in section 14, which in this example is 192.168.0.101, into the
address bar and press <return>. The 7230's Main Controls panel should be
displayed, as shown below in figure 5-6.

17) Save the address as a favorite, bookmark, or shortcut so that you can quickly

With this configuration you should always be able to access the 7230 again in future

Figure 5-6, Model 7230 Main Controls Panel

reach the model 7230 again.

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Chapter 5, WEB CONTROL PANEL OPERATION

if you are connected to the same wireless network, simply by selecting the favorite,
bookmark, or shortcut. This is because the DHCP server in the wireless router will
generally allocate the same IP address to the model 7230 each time they are both
powered up and connected.

As with the wired connection described in section 5.2.01, the iPad or other wireless
device will have no general internet access while it is connected to the wireless
router to which the 7230 is wired, but can of course switch to another wireless
network to reach the internet, although obviously at that point losing connection to
the model 7230.

5.2.03 Wired Connection to a Company or Corporate
Network Using a Static IP Address

Consult your network administrator to see if they can allocate a static IP address on
the network for the model 7230. If this is possible, then use the following procedure
to program this address into the model 7230 – this must be done before connecting
the instrument to the network.

The procedure requires the use of a computer with an Ethernet 10 or 100 Base T
adaptor with RJ45 connector set to support TCP/IP protocol with an installed web
browser. As an example, a Windows 7 PC with Internet Explorer 8 is suitable, but so
are many other computer systems.

1) Close all open programs on the computer and unplug any existing network
connection.

2) Plug one end of the supplied RJ45 patch cord to the computer and the other end
into the LAN connector on the rear panel of the model 7230.

3) With the rear-panel mounted power switch set to 0 (off), plug the line cord into
the model PS0110 power supply unit and the 5-pin DIN plug on the power cable
from the PS0110 to the 7230’s rear panel POWER INPUT connector.

4) Set the rear panel Config 2 switch to position 0

5) Turn on the model 7230. The front panel status light should be green.

6) Open a browser session on the computer. Since there is no connection to the
internet you will not see the normal opening page, but an error message. If using
Internet Explorer the message will be as shown in figure 5-7

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Chapter 5, WEB CONTROL PANEL OPERATION

Figure 5-7, Initial Browser Window

8) Type 169.254.150.230 into the address bar and press <return>. The 7230's Main
Controls panel should be displayed, as shown below in figure 5-8.

9) Click the Equations tab, to show the Equations panel, as shown below in

Figure 5-8, Model 7230 Main Controls Panel

figure 5-9

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Chapter 5, WEB CONTROL PANEL OPERATION

Figure 5-9, Model 7230 Equations Panel

10) In the Command input box type
IPADDR XXX XXX XXX XXX
where XXX XXX XXX XXX is the static IP address given to you by your

network administrator. Note that there are no periods (full stops) between the
digit groups. So if, for example, you were given an IP address of 10.150.11.2,
type IPADDR 10 150 11 2 as shown in figure 5-10, and press <return>.

11) Once this is done, communications will immediately be lost and an error
message, similar to that shown in figure 5-11, will be shown:

5-8

Figure 5-10, Entering New IP Address

Chapter 5, WEB CONTROL PANEL OPERATION

Figure 5-11, Communication Error Message

12) Now unplug the network connection linking the model 7230 and the computer.

Connect the 7230’s LAN connector to the company or corporate network.
Similarly, connect the computer back to the corporate network.

13) Open a browser session and use the saved a shortcut, bookmark, or favorite to
verify that you can access the model 7230.

With this configuration you should always be able to access the 7230 again in future
simply by selecting the favorite, bookmark, or shortcut. In addition, of course, you
will also have access to the internet, and the model 7230 can be controlled from any
computer on the same network. If the network has wireless access points then it will
also be possible to control the model 7230 from wireless devices.

5.2.04 Wired Connection to a Company or Corporate
Network Using a DHCP Allocated IP Address

Connecting the model 7230 to a company or corporate network using that network’s
DHCP server involves no effort from the corporate or company IT department in
allocating a fixed IP address, but consequently can require a little more effort by the
end user.

1) Connect the LAN connector on the rear panel of the model 7230 to an active
network connection on the company or corporate network

2) Set the rear panel Config 2 switch to position 1. This will cause the model 7230
to be allocated an IP address by the DHCP server in company network.

3) With the rear-panel mounted power switch set to 0 (off), plug the line cord into
the model PS0110 power supply unit and the 5-pin DIN plug on the power cable
from the PS0110 to the 7230’s rear panel POWER INPUT connector.

4) Turn on the model 7230. The front panel status light should initially be green,

but will then flash red and green as it “counts out” the IP address it has been

allocated by the network DHCP server.

5) Most company and corporate networks use a domain controller, but the model

7230 cannot log onto a domain and so you will not in general be able to “see” it

by browsing network resources. However, once the allocated IP address is

known you will be able to access the instrument by typing this into a browser’s

address bar.

There are several possible ways of identifying the IP address

Using SRInstFinder

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Chapter 5, WEB CONTROL PANEL OPERATION

This is a simple utility for a PC which searches for any model 7230's on the
network. It can be downloaded from the www.signalrecovery.com website.
When run, it displays the dialog shown in figure 5-12 below

Figure 5-12, SRInstFinder Utility

Click the Find Instruments button. If any model 7230’s are found on the network,

the Instrument field will be updated, as shown in Figure 5-13.

Figure 5-13, SRInstFinder Utility, Model 7230S/N 11123825

found at IP Address 192.168.11.21

If there are several instruments then the up/down buttons allow you to scroll

through them until the required unit is found. The Instrument field displays the
stored instrument name (see command NAME in section 6.7.12) that can be used
in addition to the serial number to aid identification. In the case of figure 5-13,
the name is “Optical Lab”.

Click the IP address hyperlink to access the model 7230. This will open your

default browser and display the model 7230’s Main Controls page. If you check

the “Close program…” box then the SRInstFinder utility will also be closed

automatically.

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Chapter 5, WEB CONTROL PANEL OPERATION

Use the FindLockin Page on the www.signalrecovery.com website.

This page, accessible via a link from the Model 7230 page on the website, will

search for model 7230’s on the same network where their IP address shares the

same first three octets as the computer from which it is being run. This will
typically be the case where the controlling computer is in the same location or
department as the model 7230 (there can be up to 255 devices with IP addresses
that share the first three octets) so in most cases this will not be a limitation.

Count the Status Light Flashes

If neither of the above methods will work, then it is possible to count the flashes
of the front panel STATUS light, in accordance with the information in section

4.1.04

6) Once the IP address for the 7230 has been found, type it into the browser's
address bar and press <return>. The 7230's Main Controls panel should be
displayed.

7) Save the address as a shortcut, bookmark, or favorite.

Using this configuration you should be able to access the 7230 again in future simply
by selecting the favorite, bookmark, or shortcut. However, depending on the policy
of the DHCP server, and the length of leases that it grants, it may be that if the model
7230 is powered down and then turned back on again after a delay (e.g. over a
weekend) it may be allocated a different IP address by the DHCP server.

However, many DHCP servers keep a table of past IP address assignments, so that
they can preferentially assign to a client (such as the model 7230) the same IP
address that the client previously had. In this case the 7230 will normally get the
same address.

If, however, the address changes, then you will need to repeat the above procedure to
find the new address. One way of avoiding having to do this is, of course, to leave
the model 7230 permanently switched on and connected to the network.

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Chapter 5, WEB CONTROL PANEL OPERATION

5.3 Web Control Panels

5.3.01 Main Controls: Overview

Figure 5-14, Main Controls Panel

The Main Controls panel, shown above in Figure 5-14, is displayed when a browser
session is opened on the instrument’s IP address. There are four digital indicators
along the bottom of the screen, and controls for all the most common functions in the
upper section.

There are several ways of operating the controls. Some, such as the Sensitivity
control, have a range of fixed settings. To adjust these, simply click on the control
and then click the required setting (see figure 5-15).

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Chapter 5, WEB CONTROL PANEL OPERATION

Figure 5-15, Adjusting Sensitivity Control

Other controls, such as the Phase and Oscillator Frequency controls, are adjusted by
typing the required value directly into a text box.

Figure 5-16, Adjusting Phase Control

To change the units applying to the digital indicators between “%” (percentage of

full scale sensitivity) and calibrated units (e.g. mV or mA), click the Units indicator,
as shown in figure 5.17.

Figure 5-17, Switching between % and mV Units

Operation of radio button and check boxes is the same as on other web pages.
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

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Chapter 5, WEB CONTROL PANEL OPERATION

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 7230 can make simultaneous measurements at two
different reference frequencies, internal or external. If using two external references,
one of them is restricted to being no greater than 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
reference mode but with the input to the second set of demodulators taken as the X­channel output from the first, allows the 7230 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.

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Chapter 5, WEB CONTROL PANEL OPERATION

5.3.02 Main Controls: Display Indicators

Figure 5-18, Selecting Instrument Outputs

Each of the four digital indicators along the bottom of the screen can be set to
display the following instrument outputs (see figure 5-18).

Output Description

X1 X1 channel output. In Single Reference mode this is the X output
Y1 Y1 channel output. In Single Reference mode this is the Y output
Mag1 Resultant (Magnitude1) output. In Single Reference mode this is the Mag

output

Phase1 Phase1 output. In Single Reference mode this is the Phase output
Noise1 Noise output for the first demodulator.
X2 X2 channel output, valid only in when demodulator 2 is active
Y2 Y2 channel output, valid only in when demodulator 2 is active
Mag2 Resultant (Magnitude2) output, valid only in when demodulator 2 is

active

Phase2 Phase2 output, valid only in when demodulator 2 is active
Eqn1 Output value of User Equation 1
Eqn2 Output value of User Equation 2

If an output is in overload, the display is shown in red text (figure 5-19).

Figure 5-19, Output Overload Indication

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Chapter 5, WEB CONTROL PANEL OPERATION

5.3.03 Main Controls: Input

Figure 5-20, Main Controls: Input

The Input controls, shown in figure 5-20, affect the signal channel.

AC Gain

The AC gain control allows the AC gain to be adjusted from 0 dB to 90 dB in 6 dB
steps, although not all settings are available at all full-scale sensitivity settings.

To obtain the best accuracy, use the highest value of AC Gain that is possible
without causing signal input overload, indicated by bit 6 in the Status Byte (see
section 5.3.11) being red. The Input Limit value, displayed in the Status panel, is the
zero to peak signal that may be applied without causing signal overload.

Input mode
This control sets the preamplifier input configuration to either voltage or current
mode.

A (voltage)

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

-B (voltage)

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

A-B (voltage)

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).

B (current^6)

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

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Chapter 5, WEB CONTROL PANEL OPERATION

bandwidth current-to-voltage converter (10E6 V/A transimpedance).

B (current^8)

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 (10E8 V/A transimpedance).

Off

In this setting, used for test purposes, the signal channel input is disconnected.

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

Input device

This control selects the type of input device used for the signal channel connections
in voltage mode input settings.

FET

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

Bipolar

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.

Input shield

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

Float

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

Ground

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

Line filter
This control selects the mode of operation of the line frequency rejection filter and
offers the following settings:-

Selection Function

50 Hz Enable 50 Hz notch filter
100 Hz Enable 100 Hz notch filter
50Hz + 100Hz Enable 50 and 100 Hz notch filters

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Chapter 5, WEB CONTROL PANEL OPERATION

60Hz Enable 60 Hz notch filter
120Hz Enable 120 Hz notch filter
60Hz + 120Hz Enable 60 and 120 Hz notch filters

Demod2 source

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

Sig Chan ADC

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.

Rear ADC1

In this setting the signal input to the second set of demodulators is taken from the
rear panel auxiliary input 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 output (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

5.3.04 Main Controls: Reference 1

Figure 5-21, Main Controls: Reference 1

The Reference 1 controls, figure 5-21, affect the reference channel of the first set of
demodulators.

Source

This control allows selection of the source of reference signal used to drive the
reference 1 circuitry, and has four settings:-

Internal

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.

External digital

In this setting the reference channel is configured to accept a TTL reference
source applied to the front panel REF IN input connector, except if using the
Dual Demodulator mode and the Reference 2 input selection is set to External

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Chapter 5, WEB CONTROL PANEL OPERATION

Analog. In this case a TTL reference at a frequency no greater than 3.0 kHz
should be applied to the rear panel TRIG IN connector.

External analog

In this setting the reference channel is configured to accept an analog reference
source applied to the front panel REF IN input connector

Virtual

This activates the virtual reference mode, and the Main Controls page is
reconfigured to support the additional controls that are required (figure 5-22)

Figure 5-22, Virtual Reference Controls

The virtual reference mode is very similar to the single reference mode with

internal reference, and is the simplest of the three additional modes of operation.

NOTE: This mode is only suitable for signals at frequencies between 100 Hz

and the upper frequency limit of the instrument (120 or 250 kHz).

The controls operate as follows:-

Start Freq

This control defines the start frequency from which the instrument will begin to
try to find the applied signal. It should be set to a value close to, but less than,
the expected signal frequency.

Stop Freq

This control defines the stop frequency at which the instrument abandons its
search for the signal, and should be set to a value greater than the expected signal
frequency.

Step Size

This control defines the frequency step size used during the automatic search.

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Chapter 5, WEB CONTROL PANEL OPERATION

Time/Step

This control defines the time per point within the automatic search.

Search

Clicking the Search button starts the internal oscillator at the defined start
frequency and increases it at the selected step size and rate. If the instrument
detects a signal greater than 50% of full-scale then the search stops.

Frequency

This indicator shows the current frequency at which the Reference 1 channel is
operating.

Harmonic

This control allows selection of the harmonic of the reference frequency at which the
reference 1 channel of the lock-in amplifier operates. It can be set to any value
between 1F and 127F, but most commonly is set to 1F.

Phase

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

+90 Deg

Each click of this button adds 90° to the reference 1 phase setting.

Auto Phase

Clicking this button initiates an Auto-Phase operation, where the value of the signal
phase with respect to the reference is computed and an appropriate phase-shift is
then introduced into the reference 1 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.

5.3.05 Main Controls: Oscillator

Figure 5-23, Oscillator Controls

The Oscillator controls, shown in figure 5-23, affect the internal oscillator.

Amplitude

This control sets the amplitude of the signal at the front panel OSC OUT connector
to any value between 1 mV and 5 V rms.

Frequency

The frequency of the instrument's internal oscillator may be set, using this control, to
any value between 0.001 Hz and 120.000 kHz (or 250.000 kHz if the instrument is
fitted with the 7230/99 option) with a 1 mHz resolution.

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Chapter 5, WEB CONTROL PANEL OPERATION

10

2

DR 20 log ACGain (in dB)

SEN



  




10

2

DR 20 log 12

0.002



  




5.3.06 Main Controls: Output 1

Figure 5-24, Output 1 Controls

The Output 1 controls, shown in figure 5-24, affect the first set of demodulators

Sensitivity

When set to voltage input mode, using the control in the Input section, the
instrument's full-scale voltage sensitivity may be set to any value between 10 nV and
1 V in a 1-2-5 sequence.

When set to current input mode, using the control in the Input section, the
instrument's full-scale current sensitivity may be set to any value between 10 fA and
1 µA (wide bandwidth 10^6 mode), or 10 fA and 10 nA (low-noise 10^8 mode), in a
1-2-5 sequence.

The Dynamic Reserve figure, expressed in decibels, as calculated by the following
equation and shown on the display in the Status block:-

Example:­If AC Gain = 12 dB and SEN = 2 mV then

DR = 48 dB

Auto Sen

This button operates when the reference frequency is above 1 Hz. A single Auto­Sensitivity 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.

Time Constant

The time constant of the output filters is set using this control. When the Fast control
in the Output Filters section is checked, the range is 10 µs to 100 ks in a 1-2-5
sequence; when it is not checked the minimum time constant is 5 ms.

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Chapter 5, WEB CONTROL PANEL OPERATION

Slope

The roll-off of the output filters is set, using this control, to values 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.

X Offset and Y Offset
These controls allow manual adjustment of the X1 channel and Y1 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 X1 channel or Y1 channel output when
the corresponding check boxes are checked.

The values are also set automatically and turned on by the Auto-Offset function,
which is activated by clicking the Auto Offset button.

Auto Offset

This button adjusts the X1 offset and Y1 offset values so that the X1 channel and Y1
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.

Noise Measurement

This control is used to configure the instrument for noise measurements. When
checked, 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 is turned
off.

The output filter slope is also restricted to either 6 or 12 dB/octave.

When the control is unchecked then the instrument is configured for normal lock-in
amplifier operation.

Noise Buffer LengthThis control, which is only active when the above Noise
Measurement control is checked, 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 be selected.

The operation of this feature is described in more detail in section 3.3.20

5.3.07 Main Controls: Reference 2

Figure 5-25, Main Controls: Reference 2

When the Demodulator is set to Dual mode, the Reference 2 controls section of the
panel, shown in figure 5-25, is active. These controls affect the second set of
demodulators.

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Chapter 5, WEB CONTROL PANEL OPERATION

Source

This control allows selection of the source of reference signal used to drive the
reference 2 circuitry, and has three settings. The corresponding control in the
Reference 1 section also offers the same choices when in dual mode:-

Internal

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.

External digital

In this setting the reference channel is configured to accept a TTL reference
source applied to the front panel REF IN input connector, except if the
Reference 1 input selection is set to External Analog, in which case a TTL
reference at a frequency no greater than 3.0 kHz should be applied to the rear
panel TRIG IN connector.

External analog

In this setting the reference channel is configured to accept an analog reference
source applied to the front panel REF IN input connector

Note that it is not possible to use virtual reference mode when the instrument is in
dual mode, so there is no selection to allow this.

Frequency

This indicator shows the current frequency at which the Reference 2 channel is
operating.

Harmonic

This control allows selection of the harmonic of the reference frequency at which the
reference 2 channel of the lock-in amplifier operates. It can be set to any value
between 1F and 127F, but most commonly is set to 1F.

Phase

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

+90 Deg

Each click of this button adds 90° to the reference 2 phase setting.

Auto Phase

Clicking this button initiates an Auto-Phase operation, where the value of the signal
phase with respect to the reference is computed and an appropriate phase-shift is
then introduced into the reference 2 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.

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Chapter 5, WEB CONTROL PANEL OPERATION

5.3.08 Main Controls: Output 2

The Output 2 controls, shown in figure 5-26, affect the second set of demodulators

Sensitivity

When set to voltage input mode, using the control in the Input section, the
instrument's full-scale voltage sensitivity may be set to any value between 10 nV and
1 V in a 1-2-5 sequence.

When set to current input mode, using the control in the Input section, the
instrument's full-scale current sensitivity may be set to any value between 10 fA and
1 µA (wide bandwidth 10^6 mode), or 10 fA and 10 nA (low-noise 10^8 mode), in a
1-2-5 sequence.

Figure 5-26, Output 2 Controls

Auto Sen

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.

Time Constant

The time constant of the output filters is set using this control. When the Fast control
in the Output Filters section is checked, the range is 10 µs to 100 ks in a 1-2-5
sequence; when it is not checked the minimum time constant is 5 ms.

Slope

The roll-off of the output filters is set, using this control, to values 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.

X Offset and Y Offset
These controls allow manual adjustment of the X2 channel and Y2 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 X2 channel or Y2 channel output when
the corresponding check boxes are checked.

The values are also set automatically and turned on by the Auto-Offset function,
which is activated by clicking the Auto Offset button.

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Chapter 5, WEB CONTROL PANEL OPERATION

Auto Offset

This control adjusts the X2 offset and Y2 offset values so that the X2 channel and
Y2 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.

5.3.09 Main Controls: Output Filters

Figure 5-27, Output 2 Controls

The Output Filters controls, shown in figure 5-27, affect both sets of demodulators.

Synchronous

When this box is unchecked, which is the normal mode, time constants are not
related to the reference frequency period. When it is checked and the reference
frequency is 10 Hz or lower, the actual time constant used is chosen to be some
multiple of the reference frequency period, giving a much more stable output at low
frequencies than would otherwise be the case. Note that in this mode, depending on
the reference frequency, output time constants shorter than 100 ms cannot be used.

Fast mode

When this box is unchecked, the instrument’s 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.

When it is checked, 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.

5.3.10 Main Controls: Demodulator Control

Figure 5-28, Demodulator Control

The Demodulator control, shown in figure 5-28, determines whether the second set
of demodulators is active.

Single

When selected the instrument functions as a conventional dual phase lock-in
amplifier, and only the first set of demodulators is active. It includes both internal
and external reference modes and provides detection either at the reference
frequency or one harmonic of it. Virtual Reference mode can only be used when in
Single Reference mode.

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Chapter 5, WEB CONTROL PANEL OPERATION

Dual

When selected both sets of demodulators are active, and can be used in the following
ways:

Dual Reference

In dual reference mode the model 7230 can make simultaneous measurements at
two different reference frequencies. These can be internal or external, subject to
the limitation that if two external references are used, one of them cannot be
greater than 3.0 kHz. The Reference controls under Reference 1 and Reference 2
are used to select the source of reference for each demodulator, and for this
mode will be set to different sources.

Dual Channel Detection

Normally the signal input in dual reference mode is common to both
demodulators, and uses the main signal channel. However, it is possible to use
the auxiliary ADC 1 input as the signal input to the second set of demodulators,
allowing the instrument to function as two independent lock-in amplifiers, Note,
however, that in this mode there is no adjustable AC Gain in the ADC 1 input
channel so the range of full-scale sensitivity settings for the second set of
demodulators is limited.

Dual Harmonic

Dual harmonic mode allows the simultaneous measurement of two different
harmonics of the input signal. The Reference controls under Reference 1 and
Reference 2 are used to select the source of reference for each demodulator, and
for this mode will be set to the same source, with each Harmonic control set to a
different value.

Tandem Demodulation

This is essentially the same as dual reference mode but with the input to the
second set of demodulators taken not from the main signal channel or ADC 1
input but from the X-channel output from the first set of demodulators. It allows
the instrument to detect the amplitude of a low frequency modulation of a
higher-frequency carrier signal.

5.3.11 Main Controls: Status Indicators

Figure 5-29, Status Indicators

The Status section gives a quick overview of the instrument status.

Input limit

This is the zero to peak value above which the instrument will be in input overload,
based on the present AC gain setting.

Dynamic reserve

This is the calculated level of Dynamic Reserve based on the present sensitivity and
AC gain settings.

Status byte and Overload byte

These two arrays of LED’s show the bit values in these two bytes, which are defined

5-26

Chapter 5, WEB CONTROL PANEL OPERATION

in the following table:
Bit Status Byte Overload Byte

bit 0 command complete X(1) output overload
bit 1 invalid command Y(1) output overload
bit 2 command parameter error X(2) output overload
bit 3 reference unlock Y(2) output overload
bit 4 output overload CH1 output overload
bit 5 new ADC values available CH2 output overload
after trigger
bit 6 input overload CH3 output overload
bit 7 data available CH4 output overload

Table 5-1, Status and Overload Byte Bit Definitions

The LEDs are green when the corresponding bit is asserted and when this is a “no

fault” condition, and red when it is asserted but this indicates a fault. Bit 0 in each

case is on the right and bit 7 on the left.
If the mouse cursor is hovered over a bit then the bit function is indicated in a pop up

box. In Figure 5-29, for example, the cursor has been hovered over bit 3 in the Status
byte; the fact that this is red indicates a loss of reference lock, while bit 1 indicates
“command done” and bit 5 that the auxiliary ADC inputs have been triggered.

This completes the description of the Main Controls panel.

5.3.12 Oscillator: Overview

Figure 5-30, Oscillator Panel

The Oscillator panel, shown above in Figure 5-30, gives access to the full set of
controls for the internal oscillator. Note, however, that the basic controls setting its
amplitude and frequency are also displayed on the Main Controls panel.

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Chapter 5, WEB CONTROL PANEL OPERATION

5.3.13 Oscillator: Sweep Control

Figure 5-31, Oscillator Panel, Sweep Controls

The internal oscillator output in the model 7230 can be swept in frequency or
amplitude, or in both parameters, using these controls.

Time/Step

This control defines the time per point during a frequency or amplitude sweep

Start

This button is used to start or stop a frequency and/or amplitude sweep. The Armed
checkbox in the required sweep type controls must also be checked in order to run a
sweep.

Frequency Sweep group
Start frequency

This control defines the start frequency from which the instrument will start a
frequency sweep.

Stop frequency

This control defines the end frequency for the frequency sweep

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.

Armed

If this box is checked then the defined sweep will be run when the Start button is
clicked.

Linear

When this radio button is selected, all frequency steps are equal and the
frequency increases linearly with time.

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Chapter 5, WEB CONTROL PANEL OPERATION

Logarithmic

When this radio button is selected, 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:-

1000.000 Hz

1100.000 Hz

1210.000 Hz

1331.000 Hz

1464.100 Hz

1610.510 Hz

1771.561 Hz

1948.717 Hz

2000.000 Hz

Amplitude Sweep group
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

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

Step size

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.

Armed

If this box is checked then the defined sweep will be run when the Start button is
clicked. If Start Amplitude is greater than Stop Amplitude then the oscillator
amplitude will decrease with time.

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Chapter 5, WEB CONTROL PANEL OPERATION

5.3.14 Oscillator: Modulation Control

Figure 5-32, Oscillator Panel, Modulation Controls

The internal oscillator output in the model 7230 can be amplitude or frequency
modulated using an external control voltage using the controls on this panel, shown
on figure 5-32.

Frequency (VCO) group
Enable FM

When checked, this control turns the frequency modulation on; when cleared, it
turns it off.

Center frequency

This control may be set to any value between 0.001 Hz and 120.000 or

250.000 kHz.

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 centre frequency.

Note: The center frequency  the span frequency must not exceed the

frequency limits of the oscillator (0 to 120 or 250 kHz). Hence the center
frequency plus the span frequency cannot exceed 120 or 250.0 kHz and the
center frequency must be greater than or equal to the span frequency. For
example, for an instrument fitted with the 7230/99 option 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..

Center voltage

This control sets the input voltage corresponding to the centre frequency.

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Chapter 5, WEB CONTROL PANEL OPERATION

Voltage span

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

Filter control

The FM 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.

Amplitude group
Enable AM

When checked, this control turns the amplitude modulation on; when cleared, it
turns it off.

ADC1 as source and Ext ref as source

These radio buttons set the source of the signal that is used for amplitude
modulation. When set to ADC1 as source, the signal should be applied to the
rear panel ADC1 input, and the Center voltage and Voltage span controls are
active.

When set to Ext ref as source the control signal should be applied to the

external reference channel REF IN input. 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.

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.

Voltage span

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

Filter control

The AM 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.

5.3.15 Oscillator: Amplitude and Frequency Controls

These two controls duplicate the controls on the Main Controls panel.

Amplitude

This control sets the amplitude of the signal at the front panel OSC OUT connector
to any value between 1 µV and 5 V rms.

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Chapter 5, WEB CONTROL PANEL OPERATION

Frequency

The frequency of the instrument's internal oscillator may be set, using this control, to
any value between 0.001 Hz and 120.000 kHz (or 250.000 kHz if the instrument is
fitted with the 7230/99 option) with a 1 mHz resolution.

This completes the description of the Oscillator panel.

5.3.16 Rear Panel: Overview

Figure 5-33, Rear Panel Controls

The Rear Panel, shown above in Figure 5-33, is used to configure the instrument’s
analog outputs, to read the auxiliary ADC input voltages, to set the communications
parameters used for the RS232 interface, to set and read the digital IO port, and for
other peripheral controls.

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Chapter 5, WEB CONTROL PANEL OPERATION

5.3.17 Rear Panel: DACs

Figure 5-34, DACs Controls

These controls, shown in figure 5-34, are used to configure the signals that will
appear on the four DAC connectors on the rear panel of the instrument. Each control
has a signal selector; if this is set to User setting then the text box below is used to
specify the corresponding voltage; otherwise it is grayed out.

The controls operate as follows:
DAC1, DAC2, DAC3, and DAC4

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)

In this setting the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the X1%fs display as follows:-

X1% 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)

In this setting the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the Y1%fs display as follows:-

Y1% DAC Voltage
+300 7.5 V
+100 2.5 V
0 0.0 V

-100 -2.5 V

-300 -7.5 V

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Chapter 5, WEB CONTROL PANEL OPERATION

10 X output

RATIO

ADC1 Input










0.1

10

0.5

RATIO

1.000

RATIO 2















Magnitude% (2.5V fs)

In this setting the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the MAG1%fs display as follows:-

MAG%fs DAC Voltage
+300 7.5 V
+100 2.5 V
0 0.0 V

Phase (+9 V = +180°)

In this setting the corresponding DAC connector on the rear panel of the
instrument outputs a voltage related to the Phase 1 display as follows:-

PHA 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 Noise%fs display as follows:-

Noise%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

(Main Controls panel) must be ON.

Ratio (10.X%/ADC1)

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:-

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:-

The relationship between the voltage at the DAC connector and the RATIO

5-34

value is defined as follows:-

Chapter 5, WEB CONTROL PANEL OPERATION

10

10 X output

LOG RATIO log

ADC1 input










10

0.1

10

0.5

LOG RATIO log

1.000

LOG RATIO = 0.301













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:-

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
setting, the X1 channel output were maximized and a 1 V signal were applied to
the ADC1 input, then the value of Log ratio would be:-

The relationship between the voltage at the DAC connector and the Log ratio

value is defined as follows:-

Log ratio DAC Voltage
+2.000 2.000 V
0 0.0 V

-3.000 -3.000 V

Note: If Ratio < 0 then Log ratio = -3.000

Equation 1

Equation 1 DAC Voltage
+10000 10.0 V
0 0.0 V

-10000 -10.0 V

Equation 2

When set to Equation 1 the corresponding DAC connector on the rear panel of
the instrument outputs a voltage related to Equation 1, which is defined using the
Output Equations panel, as follows:-

When set to Equation 2 the corresponding DAC connector on the rear panel of
the instrument outputs a voltage related to Equation 2, which is defined using the
Output Equations panel, as follows:-

5-35

Chapter 5, WEB CONTROL PANEL OPERATION

Equation 2 DAC Voltage
+10000 10.0 V
0 0.0 V

-10000 -10.0 V

User setting

When User setting is selected the corresponding DAC connector on the rear
panel of the instrument outputs the voltage set by the corresponding control, in
the range 10.000 V.

Digital Sig Mon

When set to the Digital Sig Mon setting the corresponding DAC connector on
the rear panel of the instrument outputs a voltage representing the digital output
of the main signal channel ADC

Digital Sig Mon2

When set to the Digital Sig Mon2 setting the corresponding DAC connector on
the rear panel of the instrument outputs a voltage representing the digital input to
the second demodulator.

ADC1 Monitor

When set to the ADC1 Monitor setting the corresponding DAC connector on the
rear panel of the instrument outputs a voltage representing the voltage at the rear­panel ADC1 input connector.

Sync Osc

When set to the Sync Osc setting the corresponding DAC connector on the rear
panel of the instrument outputs a voltage representing the sinusoidal waveform
applied to the first in-phase demodulator. If the reference mode is external, this
allows generation of a sinusoidal signal that is phase locked, and with adjustable
phase relationship, to it.

5.3.18 Rear Panel: ADCs

The ADCs indicators show the voltages present at the instrument’s rear-panel ADC1

5-36

Figure 5-35, ADC Indicators

Chapter 5, WEB CONTROL PANEL OPERATION

to ADC4 inputs, and a radio button allows selection of two trigger modes, as
follows.

Internal trigger (1kHz)

A conversion is performed on ADC1, ADC2, ADC3 and ADC4 every 1 ms, with the
results being displayed above and being available via the computer interfaces.

External trigger (1kHz)

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 above and are available via
the computer interfaces.

5.3.19 Rear Panel: RS232

Figure 5-36, RS232 Controls

These controls, shown in figure 5-36, set the parameters which control the RS232
communications port, as follows:

Baud rate

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

75 1800
110 2000

134.5 2400

150 4800
300 9600
600 19200
1200 38400

Data bits

This control sets the data transmission to either 7 or 8 data bits

Parity

This control sets the data transmission to use no parity bit, or one bit with either even

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Chapter 5, WEB CONTROL PANEL OPERATION

or odd parity.

Delimiter

The character selected 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.

Echo on

When this box is checked, the instrument echoes 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 unchecked,,
character echo is suppressed.

Prompt on

When this box is checked, a prompt character is generated by the model 7230 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. When
unchecked, no prompt character is generated.

5.3.20 Rear Panel: Ref Mon

Figure 5-37, Ref Mon Control

When checked, this control, shown in figure 5-37, is used to configure the rear-panel
TRIG OUT connector as a TTL reference monitor output. In this setting the signal
at this connector is a TTL signal at the present reference frequency (either internal or
external) which can be used to monitor correct operation of the reference channel.

5.3.21 Rear Panel: Expand

Figure 5-38, Expand Control

These two controls, shown in figure 5-38, control the output expansion status of the
X1 and Y1 outputs when they are specified as analog signals at the DAC connectors.

X1 expand

When this box is checked and the DAC1 output is set to X% then the voltage present
at the output will be multiplied by 10. Hence if the X% output is 10% and X1 expand
is checked, the voltage at DAC1 will be 2.5 V rather than the 0.25 V that would
apply otherwise.

Y1 expand

When this box is checked and the DAC2 output is set to Y% then the voltage present
at the output will be multiplied by 10. Hence if the Y% output is 10% and Y1 expand
is checked, the voltage at DAC1 will be 2.5 V rather than the 0.25 V that would

5-38

Chapter 5, WEB CONTROL PANEL OPERATION

apply otherwise.

5.3.22 Rear Panel: Bind

Figure 5-38, Bind Control

When the bind control, shown in figure 5-38, is checked, it is only possible to
operate the instrument using the web pages from the computer at which this box was
checked. This prevents another user on the same network trying to control the same
instrument.

5.3.23 Rear Panel: Digital Port

Figure 5-39, Digital Port

The Digital Port controls and indicators, shown above in figure 5-39, allow the
operating mode of each of the eight pins of the DIGITAL I/O connector on the rear
panel of the instrument to be set and its logic status to be set and read. This port may
be used for controlling or reading the status of external equipment, for example the
switching of heaters or attenuators, via a suitable user-supplied external interface
circuit.

Each of the eight pins is configured as an input or output by the Direction buttons;
when the digit is set to OP it is an output; when set to IP it is an input.

For those bits configured as outputs, the Output value buttons define whether they
are high (value = 1) or low (value = 0); simply click the buttons to change the value.

The Input value indicators show the current logic state of all eight bits, regardless
of whether they are inputs or outputs.

5.3.24 Rear Panel: DIP switches

The DIP switch indicator, shown above in figure 5-40, displays the current position

Figure 5-40, DIP Switches

5-39

Chapter 5, WEB CONTROL PANEL OPERATION

of the three rear panel CONFIG DIP switches. the function of these switches is as
follows:

Switch 1: Retain Settings

This switch determines whether the instrument will retain its settings during power­down, according to the following table:

Position Description
0 Unit powers on with all settings set to factory default values (see
Appendix D)
1 Unit powers on with the same settings as when it was powered off

Hence to perform a reset to the factory default settings, turn off the power switch, set
switch 1 to position 0, and turn the power switch back on. Switch 1 can then be set
back to 1 if desired.

Switch 2: IP Address Control

This switch selects the mode of operation of the Ethernet interface, according to the
following table:

Position Description
0 The model 7230 uses a static IP address, as set by the command

IPADDR. This command can be sent via any one of the three interfaces,
but if used via the Ethernet interface and specifying a different address to
that currently in use, then communications will be lost.

1 Unit uses a DHCP or AutoIP allocated address. It initially tries to get an

IP address via a DHCP server but if one is not assigned then it will auto
assign an IP address and test that it does not conflict with anything else
on the network. The AutoIP address will be in the range

169.254.XXX.XXX

Switch 3: IP Address Display

When this switch is changed from 0 to 1 or from 1 to 0 then the current IP address
(static, DHCP, or Auto IP allocated) will be shown by means of flashing the front
panel STATUS light red and green.

5.3.25 Rear Panel: USB Status

Figure 5-41, USB Status

The USB Status indicator, shown above in figure 5-41, displays the current status of
the USB communications port, and, if connected, the speed at which it is operating.

5-40

Chapter 5, WEB CONTROL PANEL OPERATION













D

C B) (A

=Equation

5.3.26 Equations: Overview

Figure 5-42, Equations

The Equations panel, shown above in Figure 5-42, allows the two user equations that
are built in to the instrument to be defined, as well as giving access to a Command
Interface that allows instrument commands to be sent to the unit and the response, if
any, displayed.

5.3.27 Equations: Equation 1 and Equation 2

Figure 5-43, Equation 1

The Equation 1(shown above in figure 5-43) and Equation 2 controls are used to
define more complex calculations on the instrument outputs than are possible using
the basic ratio and log ratio options. Each equation takes the following form:-

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:-

5-41

Chapter 5, WEB CONTROL PANEL OPERATION

Variable Range
X1 ±30000
Y1 ±30000
MAG1 0 to +30000
PHA1 (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 120000000 or 250000000
(Only available in position C)
OSCF(Oscillator Frequency)
0 to 120000000 or 250000000
(Only available in position C)

In the dual modes X2, Y2, MAG2 and PHA2 variables can also be selected.
Use the up/down buttons to select the required variable settings.
The values C1 and C2 within each equation are user-defined integer constants and

are adjusted by typing the required value into the text entry boxes and pressing
<return>.

The equations are calculated using 64-bit integers to maintain full accuracy through
to the 32-bit result that can be shown on the digital indicators. Care should be taken
when 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.

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

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Chapter 5, WEB CONTROL PANEL OPERATION

5.3.28 Equations: Command Interface

Figure 5-44, Command Interface

The Command interface, shown above in figure 5-44, allows commands to be sent to
the instrument and the response displayed. Commands are listed in Chapter 6 and
Appendix E of this manual.

Type the command in the Command input box and press <return>. The response
will be shown in the Command response box. In the figure above, the command
ID was sent and the response is "7230", the instrument's model number.

5.3.29 Equations: Auto Default

Figure 5-45, Auto Default Button

Clicking the Auto Default button, shown above in figure 5-45, 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.

5.3.30 Equations: Auto Measure

Figure 5-46, Auto Measure Button

Clicking the Auto Measure button, shown above in figure 5-46, adjusts the full-scale
sensitivity so that the magnitude output lies between 30% and 90% of full-scale, and
then performs an auto-phase operation to maximize the X channel output and
minimize the Y channel output.

This completes the description of the instrument's web control panels.

5-43

Chapter 5, WEB CONTROL PANEL OPERATION

5-44

Computer Operation

6.1 Introduction

The model 7230 includes RS232, USB and Ethernet interface ports, designed to
allow it to be completely controlled from a remote computer. All of the instrument's
controls may be operated, and all of the outputs can be read, via these interfaces.

This chapter describes the capabilities of the instrument when operated remotely and
discusses how this is done. Refer to Chapter 5 for a description of how to operate the
instrument via its built in web control panels.

6.2 Capabilities

6.2.01 General

All instrument controls can be set, with the exception of the functions selected by the
rear panel CONFIG switches, and all instrument outputs may be read remotely.

6.2.02 Operation

Control of the lock-in amplifier from a computer consists of the computer sending
commands to the instrument, with it responding either by sending back some data or
by changing the setting of one of its controls. The commands and responses are
generally ASCII text strings, although transfer of data from the instrument’s curve
buffer can also be done using a binary dump of values, with each value being two
bytes long.

Chapter 6

The computer ports cannot be used simultaneously, but when a command has been
completed, the lock-in amplifier will accept the next command from any port, and
respond to that port.

6.2.03 Compound Commands

Unlike some older SIGNAL RECOVERY instruments, the model 7230 does not
support compound commands, which were two or more simple commands separated

by semicolons (ASCII 59) and terminated by a single command terminator.

6.3 RS232 Operation

6.3.01 Introduction

The RS232 interface is primarily intended to enable the lock-in amplifier to be
operated from a computer program specially written for an application, although it
can also be used in the direct, or terminal, mode. In this mode the user enters
commands on a keyboard and reads the results from the display.

The simplest way to establish this terminal mode is to connect the instrument to a
computer running a terminal emulator, such Windows HyperTerminal.

6.3.02 General Features

The RS232 interface in the model 7230 is implemented with three wires; one carries
digital transmissions from the computer to the lock-in amplifier, the second carries
digital transmissions from the lock-in amplifier to the computer and the third is the
Logic Ground to which both signals are referred. The logic levels are ±12 V referred
to Logic Ground, and the connection may be a standard RS232 cable in conjunction

6-1

Chapter 6, COMPUTER OPERATION

with a null modem or, alternatively, may be made up from low-cost general-purpose
cable. The pinout of the RS232 connectors is shown in appendix B and cable
diagrams suitable for coupling the instrument to a computer are shown in
appendix C.

The main advantages of the RS232 interface are:

1) It communicates via a serial port which is either already fitted or can be added at
low cost via an adaptor to any computer, using leads and connectors which are
available from suppliers of computer accessories or can be constructed at
minimal cost in the user's workshop.

2) It requires no more software support than is normally supplied with the
computer, for example Windows HyperTerminal.

A single RS232 transmission consists of a start bit followed by 7 or 8 data bits, an
optional parity bit, and 1 stop bit. The rate of data transfer depends on the number of
bits per second sent over the interface, usually called the baud rate. In the model
7230 the baud rate can be set to a range of different values up to 38,400,
corresponding to a minimum time of less than 0.25 ms for a single character.

Mainly for historical reasons, there are a very large number of different ways in
which RS232 communications can be implemented. Apart from the baud rate
options, there are choices of data word length (7 or 8 bits), parity check operation
(even, odd or none), and number of stop bits (1 or 2). With the exception of the
number of stop bits, which is fixed at 1, these settings may be adjusted using the
RS232 Settings controls, discussed in chapter 5. They may also be adjusted by means
of the RS command.

NOTE: In order to achieve satisfactory operation, the RS232 settings must be set
to exactly the same values in the terminal or computer as in the lock-in amplifier.

6.3.03 Choice of Baud Rate

Where the lock-in amplifier is connected to a terminal or to a computer
implementing an echo handshake, the highest available baud rate of 38,400 is
normally used if, as is usually the case, this rate is supported by the terminal or
computer. Lower baud rates may be used in order to achieve compatibility with older
equipment or where there is some special reason for reducing the communication
rate.

6.3.04 Choice of Number of Data Bits

For most transmissions, the model 7230 lock-in amplifier uses the standard ASCII
character set, containing 127 characters represented by 7-bit binary words. If an 8-bit
data word is selected, the most significant bit is set to zero on output from the lock-in
amplifier and ignored on input.

The binary dump commands, used for the fastest data transfer rates from the internal
curve buffers, require 8 bit transmission.

6.3.05 Choice of Parity Check Option

Parity checks are not required at the baud rates available in the model 7230 with
typical cable lengths of up to a few meters. Therefore no software is provided in the
model 7230 for dealing with parity errors. Where long cables are in use, it may be
advisable to make use of a lower baud rate. The result is that any of the parity check

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Chapter 6, COMPUTER OPERATION

options may be used, but the no-parity option will result in slightly faster
communication.

Where the RS232 parameters of the terminal or computer are capable of being set to
any desired value, an arbitrary choice must be made. In the model 7230 the
combination set at the factory is even parity check, 7 data bits, and one stop bit
(fixed).

6.3.06 Handshaking and Echoes

A handshake is a method of ensuring that the transmitter does not send a data byte
until the receiver is ready to receive it.

In the RS232 standard there are several control lines called handshake lines (RTS,
DTR outputs and CTS, DSR, DCD inputs) in addition to the data lines (TD output
and RD input). However, these lines are not capable of implementing the
handshaking function required by the model 7230 on a byte-by-byte basis and are not
connected in the model 7230 apart from the RTS and DTR outputs, which are
constantly asserted.

Note that some computer applications require one or more of the computer's RS232
handshake lines to be asserted. If this is the case, and if the requirement cannot be
changed by the use of a software switch, the cable may be used in conjunction with a
null modem. A null modem is an adapter that connects TD on each side through to
RD on the other side, and asserts CTS, DSR, and DCD on each side when RTS and
DTR are asserted on the opposite sides.

With most modern software there is no need to assert any RS232 handshake lines
and a simple three-wire connection can be used. The actual handshake function is
then performed by means of bytes transmitted over the interface.

The more critical handshake is the one controlling the transfer of a command from
the computer to the lock-in amplifier, because the computer typically operates much
faster than the lock-in amplifier and bytes can easily be lost if the command is sent
from a program. (Note that because of the limited speed of human typing, there is no
problem in the terminal mode.) To overcome the problem an echo handshake should
be used. This works in the following way: after receiving each byte, the lock-in
amplifier sends back an echo, that is a byte which is a copy of the one that it has just
received, to indicate that it is ready to receive the next byte. Correspondingly, the
computer should not send the next byte until it has read the echo of the previous one.
The computer can compare each byte with its echo to provide a useful check on the
validity of the communications.

Where the echo is not required, it can be suppressed by negating bit 3 in the RS232
parameter byte. The default (power-up) state of this bit is for it to be asserted.

6.3.07 Terminators

In order for communications to be successfully established between the lock-in
amplifier and the computer, it is essential that each transmission, i.e. command or
command response, is terminated in a way which is recognizable by the computer
and the lock-in amplifier as signifying the end of that transmission.

In RS232 communications, the lock-in amplifier automatically accepts either <CR>
or <CR,LF> as an input command terminator, and sends out <CR,LF> as an output
response terminator except when the noprompt bit (bit 4 in the RS232 parameter

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Chapter 6, COMPUTER OPERATION

byte) is set, in which case the terminator is <CR>. The default (power-up) state of
this bit is zero.

6.3.08 Delimiters

Most response transmissions consist of one or two numbers followed by a response
terminator. Where the response of the lock-in amplifier consists of two numbers in
succession, they are separated by a byte called a delimiter. This delimiter can be one
of several ASCII characters and is common to all interfaces; it is set via the RS232
Settings controls or by the use of the DD command. The default value is a comma.

6.3.09 Status Byte, Prompts and Overload Byte

It is useful for a controlling program to be able to quickly determine the status of the
connected instrument. Internally, this is represented by two 8-bit values, known as
the status and overload bytes.

In RS232 communications, comparatively rapid access to the status byte is provided
by the prompt character, which is sent by the lock-in amplifier at the same time as
bit 0 (indicating command complete) becomes asserted in the status byte. This
character is sent out by the lock-in amplifier after each command response (whether
or not the response includes a transmission over the interface) to indicate that the
response is finished and the instrument is ready for a new command. The prompt
takes one of two forms. If the command contained an error, either in syntax or by a
command parameter being out of range, or alternatively if an overload or reference
unlock is currently being reported by the web control panel indicators, the prompt is
a question mark character ? (ASCII 63). Otherwise the prompt is an asterisk *
(ASCII 42).

These error conditions correspond to the assertion of bits 1, 2, 3, 4 or 6 in the status
byte. When the ? prompt is received by the computer, the ST command may then be
issued in order to discover which type of fault exists and to take appropriate action.

The prompts are a rapid way of checking on the instrument status and enable a
convenient keyboard control system to be set up simply by attaching a standard
terminal, or a simple computer-based terminal emulator, to the RS232 port. Where
the prompt is not required it can be suppressed by setting the noprompt bit, bit 4 in
the RS232 parameter byte. The default (power-up) state of this bit is zero.

Because of the limited number of bits in the status byte, it can indicate that an
overload exists but cannot give more detail. An auxiliary byte, the overload byte
returned by the N command, gives details of the location of the overload.

A summary of the bit assignments in the status byte and the overload byte is given in
table 6-1 below.

6-4

Bit Status Byte Overload Byte
bit 0 command complete X(1) output overload
bit 1 invalid command Y(1) output overload
bit 2 command parameter error X(2) output overload
bit 3 reference unlock Y(2) output overload
bit 4 output overload - read CH1 output overload
overload byte to determine location
bit 5 new ADC values available CH2 output overload
after trigger
bit 6 input overload CH3 output overload
bit 7 data available CH4 output overload

6.4 USB Operation

6.4.01 Introduction

The USB interface in the instrument supports operation at both Full and High Speed
settings. It provides two bulk data transfer endpoints, with endpoint 1 being used to
send commands to the instrument, and endpoint 2 being used to receive responses
from it.

Chapter 6, COMPUTER OPERATION

Table 6-1, Status and Overload Byte Bit Definitions

Operation of the instrument via the USB is made a great deal simpler if the user
installs the relevant SIGNAL RECOVERY USB driver software. Two versions of

driver are available; the first, a bulk USB driver SRUSBXP.SYS is compatible with
several software packages supplied by SIGNAL RECOVERY, while the second,

based on National Instrument’s VISA software, allows operation both from
SIGNAL RECOVERY software and a free LabVIEW driver. The only disadvantage

of using the VISA software driver is the need to download and install the complete
VISA environment in order to use the driver.

Readers should refer to the document “USB Drivers for the Model 7124, 7230, and

7270 Lock-in Amplifiers”, available from the www.signalrecovery.com website, for
further information about installing these drivers.

6.4.02 General Features

Unlike the RS232 interface, there is no difficulty ensuring that the correct cable type
is used for USB operation. All that is required is a standard USB-A to USB-B cable,
which if the driver is correctly installed on the computer will then allow operation
without further adjustment.

6.4.03 Terminator, Status Byte, and Overload Byte

Commands sent to the instrument over the USB interface should be terminated with
a null character (ASCII 0).

Responses from the instrument consist of a string that is terminated with either one
or three bytes, depending on the setting specified by the USBTERM command – see
section 6.7.11

When USBTERM has been set to 0, the terminator is:

1) A null character (ASCII 0)

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Chapter 6, COMPUTER OPERATION

When USBTERM has been set to 1, the terminator bytes are, in the following order:

1) A null character (ASCII 0)

2) A byte representing the value of the Status Byte (table 6-1)

3) A byte representing the value of the Overload Byte (table 6-1)

In the case of commands that generate a single response, the controlling program
should send the null-terminated command string to the instrument and then read the
response bytes sent back from it until the null is detected. If USBTERM is set to 1
then two further bytes (the status and overload bytes) should also be read to complete
the transfer.

Commands that do not return data still always return the one or three terminator
bytes, allowing the controlling program to know that the command has been
implemented.

Data stored in the instrument’s curve buffer can be transferred in two ways. In the

ASCII mode, each value is terminated in a null character until the last value, which is
then terminated as above, whereas in the binary dump mode each data point occupies
two bytes, with the complete dump being terminated as above.

In either case it is necessary to know how much data will be transferred before
initiating the transfer, by sending the M (monitor) command and reading the
response. This is because in the ASCII dump mode the controlling program needs to

“count” each instance of a null character until all available points have been read,

while in the binary dump mode it needs to know exactly how many bytes to read,
since the data itself can contain null characters. Only by doing this can the program
be prevented from requesting more data from the USB endpoint than the lock-in has
actually sent.

6.4.04 Delimiters

Most response transmissions consist of one or two numbers (expressed as ASCII
text) followed by a response terminator. Where the response of the lock-in amplifier
consists of two numbers in succession, they are separated by a byte called a
delimiter. This delimiter can be one of several ASCII characters and is common to
all interfaces; it is set via the DD command.

6.5 Ethernet Operation

6.5.01 Introduction

The Ethernet interface in the instrument supports operation on 10-BaseT and
100-BaseT networks via straight through or crossover RJ45 patch cables. The
instrument includes a built-in web server allowing any browser to communicate with
it using http protocol, as well as support for direct communication via TCP/IP.

6.5.02 IP Address

The IP address of the instrument needs to be unique on the network to which it is
connected. Consult section 5.2 for a discussion of the various ways of ensuring that
this is the case.

6.5.03 Main Controls

When the instrument has a valid IP address the browser on a computer on the same

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Chapter 6, COMPUTER OPERATION

network can then access it, simply by typing the IP address in the address bar. As
discussed in section 5.4, this will open the Main Controls remote front panel.

6.5.04 Sockets

The instrument normally accepts commands sent to sockets 50000 or 50001 on its IP
address. Any response is sent back to the IP address from which the initiating
command was sent.

Clearly, if an instrument is present on a large network then it is possible that several
users may accidentally try to control the same instrument. To prevent this it is
possible to use the Bind control on the Rear Panel web control panel to restrict
access to the computer from which it is being viewed, which then becomes the only
one from which commands will be accepted.

6.5.05 Terminator, Status Byte, and Overload Byte

Commands sent to the instrument over the Ethernet interface should be terminated
with a null character (ASCII 0).

Responses from the instrument depend on whether port 50000 or 50001 is being
used, as follows:

Port 50000

Responses consist of a string that is terminated with three bytes, in the following
order:

1) A null character (ASCII 0)

2) A byte representing the value of the Status Byte (table 6-1)

3) A byte representing the value of the Overload Byte (table 6-1)
In the case of commands that generate a single response, the controlling program

should send the null-terminated command string to the instrument and then read
the response bytes sent back from it until the null is detected. Two further bytes
(the status and overload bytes) should then be read to complete the transfer.

Port 50001

Responses consist of a string that is terminated with one byte, as follows:

1) Carriage Return character (ASCII 13)
In the case of commands that generate a single response, the controlling program

should send the null-terminated command string to the instrument and then read
the response bytes sent back from it until the carriage return is detected.

Commands that do not return data still always return the one or three terminator
bytes, allowing the controlling program to know that the command has been
implemented.

Data stored in the instrument’s curve buffer can be transferred in two ways. In the

ASCII mode, each value is terminated in a null character until the last value, which is
then terminated as above, whereas in the binary dump mode each data point occupies
two bytes, with the complete dump being terminated as above.

In either case it is necessary to know how much data will be transferred before
initiating the transfer, by sending the M (monitor) command and reading the
response. This is because in the ASCII dump mode the controlling program needs to

“count” each instance of a null character until all available points have been read,

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Chapter 6, COMPUTER OPERATION

while in the binary dump mode it needs to know exactly how many bytes to read,
since the data itself can contain null characters. Only by doing this can the program
be prevented from requesting more data from the socket than the lock-in has actually
sent.

6.5.06 Delimiters

Most response transmissions consist of one or two numbers (expressed as ASCII
text) followed by a response terminator. Where the response of the lock-in amplifier
consists of two numbers in succession, they are separated by a byte called a
delimiter. This delimiter can be one of several ASCII characters and is common to
all interfaces; it is set via the DD command 6.6 Command Format

The simple commands listed in section 6.7 have one of five forms:
CMDNAME terminator

CMDNAME n terminator
CMDNAME [n] terminator
CMDNAME [n1 [n2]] terminator
CMDNAME n1 [n2] terminator

where CMDNAME is an alphanumeric string that defines the command, and n, n1, n2
are parameters separated by spaces. When n is not enclosed in square brackets it
must be supplied. [n] means that n is optional. [n1 [n2]] means that n1 is optional and
if present may optionally be followed by n2. Upper-case and lower-case characters
are equivalent. Terminator bytes are defined in section 6.3.09.

Where the command syntax includes optional parameters and the command is
sent without the optional parameters, the response consists of a transmission of
the present values of the parameter(s).

Any response transmission consists of one or more numbers followed by a response
terminator. Where the response consists of two or more numbers in succession, they
are separated by a delimiter (sections 6.3.09, 6.3.04, and 6.5.05).

Some commands have an optional floating point mode which is invoked by
appending a . (full stop) character to the end of the command and before the
parameters. This allows some parameters to be entered or read in floating point
format. The floating point output format is given below.

±1.234E±01
The number of digits between the decimal point and the exponent varies depending

on the number but is a minimum of one and a maximum of eight. The input format is
not as strict but if a decimal point is used there must be a digit before it. An exponent
is optional. The following are all legal commands for setting the oscillator frequency
to 100.1 Hz:-

OF. 100.1
OF. 1.001E2
OF. +1.001E+02
OF. 1001E-1

6-8

6.7 Command Descriptions

This section lists the commands in logical groups, so that, for example, all
commands associated with setting controls which affect the signal channel are shown
together. Appendix G gives the same list of commands but in alphabetical order.

6.7.01 Signal Channel

IMODE [n] Current/Voltage mode input selector

The value of n sets the input mode according to the following table
n Input mode

0 Current mode off - voltage mode input enabled
1 High bandwidth current mode enabled - connect signal to B (I) input connector
2 Low noise current mode enabled - connect signal to B (I) input connector

If n = 0 then the input configuration is determined by the VMODE command.
If n > 0 then current mode is enabled irrespective of the VMODE setting.

VMODE [n] Voltage input configuration

The value of n sets up the input configuration according to the following table:
n Input configuration
0 Both inputs grounded (test mode)
1 A input only
2 -B input only
3 A-B differential mode

Note that the IMODE command takes precedence over the VMODE command.

Chapter 6, COMPUTER OPERATION

DEMOD2SRC [n] Second stage demodulator signal source

In the dual reference mode, the value of n sets the source of the signal for the second
stage demodulators according to the following table:

n Signal source
0 Main signal channel ADC (i.e. both demodulators are fed the same signal)
1 ADC1 - rear panel auxiliary ADC input
2 Demodulator 1 X channel output - this is the Tandem demodulation mode

FLOAT [n] Input connector shield float/ground control

n Selection
0 Ground
1 Float (connected to ground via a 1 k resistor)

FET [n] ] FET input selection

The value of n controls the input device used at the signal input according to the
following table:

n Input device
0 Bipolar
1 FET

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Chapter 6, COMPUTER OPERATION

DCCOUPLE [n] Input coupling control

The value of n sets the input coupling mode according to the following table:
n Coupling mode
0 AC coupled
1 DC coupled

SEN [n]
SEN. Full-scale sensitivity control

The value of n sets the full-scale sensitivity according to the following table,
depending on the setting of the IMODE control:

n full-scale sensitivity
IMODE=0 IMODE=1 IMODE=2
3 10 nV 10 fA n/a
4 20 nV 20 fA n/a
5 50 nV 50 fA n/a
6 100 nV 100 fA n/a
7 200 nV 200 fA 2 fA
8 500 nV 500 fA 5 fA
9 1 µV 1 pA 10 fA
10 2 µV 2 pA 20 fA
11 5 µV 5 pA 50 fA
12 10 µV 10 pA 100 fA
13 20 µV 20 pA 200 fA
14 50 µV 50 pA 500 fA
15 100 µV 100 pA 1 pA
16 200 µV 200 pA 2 pA
17 500 µV 500 pA 5 pA
18 1 mV 1 nA 10 pA
19 2 mV 2 nA 20 pA
20 5 mV 5 nA 50 pA
21 10 mV 10 nA 100 pA
22 20 mV 20 nA 200 pA
23 50 mV 50 nA 500 pA
24 100 mV 100 nA 1 nA
25 200 mV 200 nA 2 nA
26 500 mV 500 nA 5 nA
27 1 V 1 µA 10 nA

Floating point mode can only be used for reading the sensitivity, which is reported in
volts or amps. For example, if IMODE = 0 and the sensitivity is 1 mV the command
SEN would report 18 and the command SEN. would report +1.0E-03. If IMODE was
changed to 1, SEN would still report 18 but SEN. would report +1.0E-09

AS Perform an Auto-Sensitivity operation

The instrument adjusts its full-scale sensitivity so that the magnitude output lies
between 30% and 90% of full-scale.

ASM Perform an Auto-Measure operation

The instrument adjusts its full-scale sensitivity so that the magnitude output lies
between 30% and 90% of full-scale, and then performs an auto-phase operation to

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Chapter 6, COMPUTER OPERATION

maximize the X channel output and minimize the Y channel output.

ACGAIN [n] AC Gain control

Sets the gain of the signal channel amplifier according to the following table:­n AC Gain n AC Gain

0 0 dB 8 48 dB
1 6 dB 9 54 dB
2 12 dB 10 60 dB
3 18 dB 11 66 dB
4 24 dB 12 72 dB
5 30 dB 13 78 dB
6 36 dB 14 84 dB
7 42 dB 15 90 dB

AUTOMATIC [n] AC Gain automatic control

n Status
0 AC Gain is under manual control, either using the Main Controls panel or the

ACGAIN command

1 Automatic AC Gain control is activated, with the gain being adjusted according

to the full-scale sensitivity setting

LF [n1 n2] Signal channel line frequency rejection filter control

The LF command sets the mode and frequency of the line frequency rejection
(notch) filter according to the following tables:

n1 Selection
0 Off
1 Enable 50 or 60 Hz notch filter
2 Enable 100 or 120 Hz notch filter
3 Enable both filters

n2 Notch Filter Center Frequencies
0 60 Hz (and/or 120 Hz)
1 50 Hz (and/or 100 Hz)

6.7.02 Reference Channel

REFMODE [n] Reference mode selector

The value of n sets the reference mode of the instrument according to the following
table:

n Mode
0 Single Reference / Virtual Reference mode
1 Dual Harmonic mode
2 Dual Reference mode

NOTE: When in either of the dual reference modes the command set changes to
accommodate the additional controls. These changes are detailed in section 6.7.14

IE [n] Reference channel source control (Internal/External)

In Single Reference and Dual Harmonic mode, the value of n sets the reference input
mode according to the following table:

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Chapter 6, COMPUTER OPERATION

n Selection
0 INT (internal)
1 EXT TTL (external front panel REF input)
2 EXT ANALOG (external front panel REF input)

This command is invalid in Dual Reference mode

IE1 [n] Reference channel 1 source control, Dual Reference

In Dual Reference mode, the value of n sets the reference channel 1 source according
to the following table:

n Selection
0 INT (internal)
1 EXT TTL (external front panel REF input, except if IE2 = 2, when the reference
should be applied to the rear panel TRIG IN input, and be no greater than

3.0 kHz)

2 EXT ANALOG (external front panel REF input)

This command is only valid in Dual Reference mode.

IE2 [n] Reference channel 2 source control, Dual Reference

In Dual Reference mode, the value of n sets the reference channel 2 source according
to the following table:

n Selection
0 INT (internal)
1 EXT TTL (external front panel REF input except if IE1 = 2, when the reference
should be applied to the rear panel TRIG IN input, and be no greater than

3.0 kHz)

2 EXT ANALOG (external front panel REF input)

This command is only valid in Dual Reference mode

REFN [n] Reference harmonic mode control

The value of n sets the reference channel to one of the NF modes, or restores it to the
default 1F mode. The value of n is in the range 1 to 127.

REFMON [n] Reference monitor control

The value of n sets the function of the rear panel TRIG OUT connector according to
the following table:

n Mode
0 Trig Out signal is generated by curve buffer triggering
1 Trig Out signal is a TTL signal at the reference frequency (i.e. it is a Reference

Monitor output)

REFP[.] [n] Reference phase control

In fixed point mode n sets the phase in millidegrees in the range ±360000.
In floating point mode n sets the phase in degrees.

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Chapter 6, COMPUTER OPERATION

AQN Auto-Phase (auto quadrature null)

The instrument adjusts the reference phase to maximize the X channel output and
minimize the Y channel output signals.

FRQ[.] Reference frequency meter

If the lock-in amplifier is in the external reference source modes, the FRQ command
causes the lock-in amplifier to respond with 0 if the reference channel is unlocked, or
with the reference input frequency if it is locked.

If the lock-in amplifier is in the internal reference source mode, it responds with the
frequency of the internal oscillator.

In dual reference mode it reports the frequency of the reference 1 channel.
In fixed point mode the frequency is in mHz, and in floating point mode the

frequency is in Hz.

FRQ1[.] Reference frequency meter, reference 1 channel

In dual reference modes, the FRQ1 command causes the lock-in amplifier to respond
with 0 if the reference 1 channel is unlocked, or with the reference input frequency if
it is locked.

In fixed point mode the frequency is in mHz, and in floating point mode the
frequency is in Hz.

FRQ2[.] Reference frequency meter, reference 2 channel

In dual reference modes, the FRQ2 command causes the lock-in amplifier to respond
with 0 if the reference 2 channel is unlocked, or with the reference input frequency if
it is locked.

In fixed point mode the frequency is in mHz, and in floating point mode the
frequency is in Hz.

LOCK System lock control

Updates all frequency-dependent gain and phase correction parameters.

VRLOCK [n] Virtual reference mode lock

The Seek option of the frequency sweep mode must be used before issuing this
command, for which the value of n has the following significance:

n Mode
0 Disables virtual reference mode
1 Enters virtual reference mode by enabling tracking of the signal frequency

6.7.03 Signal Channel Output Filters

NOISEMODE [n] Noise Measurement Mode control

The value of n sets the noise measurement mode according to the following table:
n Function

0 Noise measurement mode off
1 Noise measurement mode on.

When the noise measurement mode is turned on, the output filter time constant is
automatically adjusted until it lies in the range 500 µs to 10 ms inclusive, the

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Chapter 6, COMPUTER OPERATION

NOISEMODE
= 0

NOISEMODE
= 1

NOISEMODE
= 0

NOISEMODE
= 1

n

time constant

time constant

n time constant

time constant

0

10 µs

N/A

16

2 s

N/A

1

20 µs

N/A

17

5 s

N/A

2

50 µs

N/A

18

10 s

N/A

3

100 µs

N/A

19

20 s

N/A

4

200 µs

N/A

20

50 s

N/A

5

500 µs

500 µs

21

100 s

N/A

6

1 ms

1 ms

22

200 s

N/A

7

2 ms

2 ms

23

500 s

N/A

8

5 ms

5 ms

24

1 ks

N/A

9

10 ms

10 ms

25

2 ks

N/A

10

20 ms

N/A

26

5 ks

N/A

11

50 ms

N/A

27

10 ks

N/A

12

100 ms

N/A

28

20 ks

N/A

13

200 ms

N/A

29

50 ks

N/A

14

500 ms

N/A

30

100 ks

N/A

15

1 s

N/A

synchronous time constant control is turned off, the fast analog output mode is
turned on (FASTMODE 1), and the output filter slope is set to 12 dB/octave if it had
previously been 18 or 24 dB/octave.

NNBUF [n] Noise Buffer Length control

The value of n sets the noise buffer length according to the following table:
n Function

0 Noise buffer off
1 1 second noise buffer
2 2 second noise buffer
3 3 second noise buffer
4 4 second noise buffer

TC [n]
TC. Filter time constant control

The value of n sets the output filter time constant in accordance with the following
table:

N/A means that n is an illegal value under these conditions.
The TC. command is only used for reading the time constant, and reports the current

setting in seconds. Hence if a TC 12 command were sent and the noise measurement
mode was turned off, TC would report 12 and TC. would report 1.0E-01, i.e. 0.1 s or
100 ms.

SYNC [n] Synchronous time constant control

n Effect
0 Synchronous time constant disabled
1 Synchronous time constant enabled

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