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
3-3
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
3-8
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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