Model 7280
Wide Bandwidth
DSP Lock-in Amplifier
Instruction Manual
190398-A-MNL-C
Copyright © 2005 AMETEK ADVANCED MEASUREMENT TECHNOLOGY, INC
Firmware Version
The instructions in this manual apply to operation of a Model 7280 DSP Lock-in Amplifier that is fitted with
revision 5.0 or later operating firmware. Users of instruments that are fitted with earlier firmware revisions
can update them to the current revision 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
Microsoft is a registered trademark, and Windows a trademark, of Microsoft Corporation. National
Instruments is a registered trademark of National Instruments Corporation
FCC Notice
This equipment generates, uses, and can radiate radio-frequency energy and, if not installed and used in
accordance with this manual, may cause interference to radio communications. As temporarily permitted
by regulation, operation of this equipment in a residential area is likely to cause interference, in which case
the user at his own facility will be required to take whatever measures may be required to correct the
interference.
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).
Declaration of Conformity
This product conforms to EC Directives 89/336/EEC Electromagnetic Compatibility Directive, amended by
92/31/EEC and 93/68/EEC, and Low Voltage Directive 73/23/EEC amended by 93/68/EEC.
This product has been designed in conformance with the following IEC/EN standards:
EMC: BS EN55011 (1991) Group 1, Class A (CSPIR 11:1990)
BS EN50082-1 (1992):
IEC 801-2:1991
IEC 801-3:1994
IEC 801-4:1988
Safety: BS EN61010-1: 1993 (IEC 1010-1:1990+A1:1992)
Table of Contents
Table of Contents
Chapter One, Introduction
1.1 How to Use This Manual.................................................................................................................................. 1-1
1.2 What is a Lock-in Amplifier?........................................................................................................................... 1-2
1.3 Key Specifications and Benefits....................................................................................................................... 1-3
Chapter Two, Installation & Initial Checks
2.1 Installation ........................................................................................................................................................ 2-1
2.1.01 Introduction ............................................................................................................................................. 2-1
2.1.02 Rack Mounting ........................................................................................................................................ 2-1
2.1.03 Inspection ................................................................................................................................................ 2-1
2.1.04 Line Cord Plug ........................................................................................................................................ 2-1
2.1.05 Line Voltage Selection and Line Fuses ................................................................................................... 2-1
2.2 Initial Checks.................................................................................................................................................... 2-3
2.2.01 Introduction ............................................................................................................................................. 2-3
2.2.02 Procedure................................................................................................................................................. 2-3
2.3 Line Frequency Filter Adjustment.................................................................................................................... 2-6
2.3.01 Introduction ............................................................................................................................................. 2-6
2.3.02 Procedure................................................................................................................................................. 2-6
Chapter Three, Technical Description
3.1 Introduction ...................................................................................................................................................... 3-1
3.2 Operating Modes .............................................................................................................................................. 3-1
3.2.01 Introduction ............................................................................................................................................. 3-1
3.2.02 Single Reference / Dual Reference.......................................................................................................... 3-1
3.2.03 Single Harmonic / Dual Harmonic .......................................................................................................... 3-1
3.2.04 Internal / External Reference Mode......................................................................................................... 3-2
3.2.05 Virtual Reference Mode .......................................................................................................................... 3-2
3.3 Principles of Operation..................................................................................................................................... 3-2
3.3.01 Block Diagram......................................................................................................................................... 3-2
3.3.02 Signal Channel Inputs.............................................................................................................................. 3-3
3.3.03 Line Frequency Rejection Filter .............................................................................................................. 3-4
3.3.04 AC Gain and Dynamic Reserve............................................................................................................... 3-4
3.3.05 Anti-Aliasing Filter.................................................................................................................................. 3-6
3.3.06 Main Analog-to-Digital Converter .......................................................................................................... 3-7
3.3.07 Reference Channel................................................................................................................................... 3-7
3.3.08 Phase-Shifter............................................................................................................................................ 3-8
3.3.09 Internal Oscillator - General .................................................................................................................... 3-9
3.3.10 Internal Oscillator - Update Rate............................................................................................................. 3-9
3.3.11 Internal Oscillator - Frequency & Amplitude Sweeps............................................................................. 3-9
3.3.12 Demodulators......................................................................................................................................... 3-10
3.3.13 Output Processor - Output Filters.......................................................................................................... 3-11
3.3.14 Output Processor - Output Offset and Expand ...................................................................................... 3-12
3.3.15 Output Processor - Vector Magnitude and Phase .................................................................................. 3-12
3.3.16 Output Processor - Noise Measurements............................................................................................... 3-13
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TABLE OF CONTENTS
3.3.17 Auxiliary Analog Inputs and Outputs (ADCs and DACs) .................................................................... 3-14
3.3.18 Main Microprocessor - Spectral Display............................................................................................... 3-15
3.3.19 Main Microprocessor - User Settings.................................................................................................... 3-15
3.3.20 Main Microprocessor - General............................................................................................................. 3-15
3.3.21 Main Microprocessor - Auto Functions ................................................................................................ 3-15
3.4 General ........................................................................................................................................................... 3-17
3.4.01 Accuracy................................................................................................................................................ 3-17
3.4.02 Power-up Defaults................................................................................................................................. 3-18
Chapter Four, Front and Rear Panels
4.1 Front Panel ....................................................................................................................................................... 4-1
4.1.01 A and B/I Signal Input Connectors ......................................................................................................... 4-1
4.1.02 OSC OUT Connector .............................................................................................................................. 4-1
4.1.03 REF IN Connector................................................................................................................................... 4-1
4.1.04 Electroluminescent Screen ...................................................................................................................... 4-2
4.1.05 HELP Key ............................................................................................................................................... 4-5
4.1.06 MENU Key.............................................................................................................................................. 4-5
4.1.07 SELECT CONTROL Key....................................................................................................................... 4-5
4.2 Rear Panel......................................................................................................................................................... 4-6
4.2.01 Line Power Switch .................................................................................................................................. 4-6
4.2.02 Line Power Input Assembly .................................................................................................................... 4-6
4.2.03 RS232 Connector .................................................................................................................................... 4-6
4.2.04 AUX RS232 Connector........................................................................................................................... 4-6
4.2.05 GPIB Connector ...................................................................................................................................... 4-7
4.2.06 DIGITAL I/O Connector......................................................................................................................... 4-7
4.2.07 PRE-AMP POWER Connector ............................................................................................................... 4-7
4.2.08 REF MON Connector.............................................................................................................................. 4-7
4.2.09 REF TTL Connector................................................................................................................................ 4-7
4.2.10 DAC 1 and DAC 2 Connectors ............................................................................................................... 4-7
4.2.11 CH 1 and CH 2 Connectors..................................................................................................................... 4-7
4.2.12 ADC 1, ADC 2, ADC 3 and ADC 4 Connectors .................................................................................... 4-8
4.2.13 TRIG Connector ...................................................................................................................................... 4-8
4.2.14 SIG MON Connector............................................................................................................................... 4-8
Chapter Five, Front Panel Operation
5.1 Introduction ...................................................................................................................................................... 5-1
5.2 Menu Structure................................................................................................................................................. 5-2
5.3 Menu Descriptions - Single Reference Mode .................................................................................................. 5-3
5.3.01 Main Display ........................................................................................................................................... 5-3
5.3.02 Control Selection Menu........................................................................................................................... 5-5
5.3.03 Main Menu 1 ........................................................................................................................................... 5-7
5.3.04 Signal Channel Menu .............................................................................................................................. 5-8
5.3.05 Reference Channel Menu ...................................................................................................................... 5-11
5.3.06 Output Filters Menu .............................................................................................................................. 5-12
5.3.07 Output Offset & Expand Menu ............................................................................................................. 5-13
5.3.08 Output Equations Menu......................................................................................................................... 5-15
5.3.09 Oscillator Menu..................................................................................................................................... 5-16
5.3.10 Frequency Sweep Menu ........................................................................................................................ 5-17
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TABLE OF CONTENTS
5.3.11 Amplitude Sweep Menu ........................................................................................................................ 5-20
5.3.12 Auto Functions Menu ............................................................................................................................ 5-23
5.3.13 Configuration Menu .............................................................................................................................. 5-25
5.3.14 Communications Menu.......................................................................................................................... 5-28
5.3.15 RS232 Settings Menu ............................................................................................................................ 5-28
5.3.16 GPIB Settings Menu.............................................................................................................................. 5-30
5.3.17 Communications Monitor...................................................................................................................... 5-32
5.3.18 Analog Outputs Menu - Single & Virtual Reference Modes................................................................. 5-33
5.3.19 Options Menu ........................................................................................................................................ 5-38
5.3.20 Spectral Display..................................................................................................................................... 5-39
5.3.21 Main Menu 2 ......................................................................................................................................... 5-41
5.3.22 Curve Buffer Menu................................................................................................................................ 5-42
5.3.23 Curve Select Menu ................................................................................................................................ 5-45
5.3.24 Single Graph Menu................................................................................................................................ 5-46
5.3.25 Double Graph Menu .............................................................................................................................. 5-48
5.3.26 User Settings Menu ............................................................................................................................... 5-49
5.3.27 Auxiliary I/O Menu ............................................................................................................................... 5-50
5.3.28 Digital Port Menu .................................................................................................................................. 5-53
5.4 Menu Descriptions - Virtual Reference Mode................................................................................................ 5-54
5.4.01 Virtual Reference Menus....................................................................................................................... 5-54
5.4.02 Main Display - Virtual Reference Mode ............................................................................................... 5-56
5.4.03 Configuration Menu - Virtual Reference Mode .................................................................................... 5-57
5.5 Menu Descriptions - Dual Reference Mode ................................................................................................... 5-57
5.5.01 Dual Reference Setup Menu.................................................................................................................. 5-57
5.5.02 Dual Reference Main Display ............................................................................................................... 5-58
5.5.03 Reference Channel Menu ...................................................................................................................... 5-62
5.5.04 Dual reference Output Filters Menu...................................................................................................... 5-64
5.5.05 Output Offset Ref 1 Menu ..................................................................................................................... 5-65
5.5.06 Output Offset Ref 2 Menu ..................................................................................................................... 5-66
5.5.07 Auto Functions Menu ............................................................................................................................ 5-67
5.5.08 Configuration Menu - Dual Reference Mode........................................................................................ 5-68
5.5.09 Analog Outputs Menu - Dual Reference and Dual Harmonic Modes................................................... 5-68
5.6 Menu Descriptions - Dual Harmonic Mode ................................................................................................... 5-75
5.6.01 Dual Harmonic Setup Menu .................................................................................................................. 5-75
5.6.02 Dual Harmonic Main Display................................................................................................................ 5-76
5.6.03 Reference Channel Menu ...................................................................................................................... 5-80
5.6.04 Dual Harmonic Output Filters Menu..................................................................................................... 5-82
5.6.05 Output Offset Harm 1 Menu.................................................................................................................. 5-83
5.6.06 Output Offset Harm 2 Menu.................................................................................................................. 5-84
5.6.07 Auto Functions Menu ............................................................................................................................ 5-85
5.6.08 Configuration Menu - Dual Harmonic Mode ........................................................................................ 5-86
5.7 Typical Lock-in Amplifier Experiment.......................................................................................................... 5-87
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TABLE OF CONTENTS
Chapter Six, Computer Operation
6.1 Introduction ...................................................................................................................................................... 6-1
6.2 Capabilities....................................................................................................................................................... 6-1
6.2.01 General .................................................................................................................................................... 6-1
6.2.02 Curve Storage.......................................................................................................................................... 6-1
6.2.03 Curve Display.......................................................................................................................................... 6-1
6.3 RS232 and GPIB Operation ............................................................................................................................. 6-2
6.3.01 Introduction ............................................................................................................................................. 6-2
6.3.02 RS232 Interface - General Features ........................................................................................................ 6-2
6.3.03 Choice of Baud Rate................................................................................................................................ 6-3
6.3.04 Choice of Number of Data Bits ............................................................................................................... 6-3
6.3.05 Choice of Parity Check Option ............................................................................................................... 6-3
6.3.06 Auxiliary RS232 Interface....................................................................................................................... 6-4
6.3.07 GPIB Interface - General Features .......................................................................................................... 6-4
6.3.08 Handshaking and Echoes......................................................................................................................... 6-5
6.3.09 Terminators ............................................................................................................................................. 6-6
6.3.10 Command Format.................................................................................................................................... 6-6
6.3.11 Delimiters ................................................................................................................................................ 6-7
6.3.12 Compound Commands ............................................................................................................................ 6-7
6.3.13 Status Byte, Prompts and Overload Byte ................................................................................................ 6-7
6.3.14 Service Requests...................................................................................................................................... 6-9
6.3.15 Communication Monitor Menu............................................................................................................... 6-9
6.4 Command Descriptions .................................................................................................................................... 6-9
6.4.01 Signal Channel ...................................................................................................................................... 6-10
6.4.02 Reference Channel ................................................................................................................................ 6-12
6.4.03 Signal Channel Output Filters ............................................................................................................... 6-14
6.4.04 Signal Channel Output Amplifiers ........................................................................................................ 6-16
6.4.05 Instrument Outputs ................................................................................................................................ 6-18
6.4.06 Internal Oscillator.................................................................................................................................. 6-21
6.4.07 Auxiliary Outputs.................................................................................................................................. 6-23
6.4.08 Auxiliary Inputs..................................................................................................................................... 6-24
6.4.09 Output Data Curve Buffer ..................................................................................................................... 6-26
6.4.10 Computer Interfaces (RS232 and GPIB)............................................................................................... 6-31
6.4.11 Instrument Identification ....................................................................................................................... 6-33
6.4.12 Front Panel ............................................................................................................................................ 6-33
6.4.13 Auto Default.......................................................................................................................................... 6-33
6.4.14 Dual Mode Commands.......................................................................................................................... 6-34
6.5 Programming Examples ................................................................................................................................. 6-36
6.5.01 Introduction ........................................................................................................................................... 6-36
6.5.02 Basic Signal Recovery........................................................................................................................... 6-36
6.5.03 Frequency Response Measurement ....................................................................................................... 6-36
6.5.04 X and Y Output Curve Storage Measurement....................................................................................... 6-37
6.5.05 Transient Recorder ................................................................................................................................ 6-38
6.5.06 Frequency Response Measurement using Curve Storage and Frequency Sweep ................................. 6-38
iv
TABLE OF CONTENTS
Appendix A, Specifications
Appendix B, Pinouts
B1 RS232 Connector Pinout .................................................................................................................................B-1
B2 Preamplifier Power Connector Pinout ..............................................................................................................B-1
B3 Digital Output Port Connector..........................................................................................................................B-2
Appendix C, Demonstration Programs
C1 Simple Terminal Emulator................................................................................................................................C-1
C2 RS232 Control Program with Handshakes .......................................................................................................C-1
C3 GPIB User Interface Program ...........................................................................................................................C-3
Appendix D, Cable Diagrams
D1 RS232 Cable Diagrams......................................................................................................................................D1
Appendix E, Default Settings
Auto Default Function............................................................................................................................................. E1
Appendix F, Alphabetical Listing of Commands
Index
Warranty
...................................................................................................................................... End of Manual
v
TABLE OF CONTENTS
vi
Introduction
1.1 How to Use This Manual
This manual gives detailed instructions for setting up and operating the
SIGNAL RECOVERY Model 7280 Digital Signal Processing (DSP) dual phase,
wide bandwidth, lock-in amplifier. It is split into the following chapters:-
Chapter 1 - Introduction
Provides an introduction to the manual, briefly describes what a lock-in amplifier is
and the types of measurements it may be used for, and lists the major specifications
of the model 7280.
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 1
Chapter 4 - Front and Rear Panels
Describes the instrument’s connectors, controls and indicators as referred to in the
subsequent chapters.
Chapter 5 - Front Panel Operation
Describes the capabilities of the instrument when used as a manually operated unit,
and shows how to operate it using the front panel controls.
Chapter 6 - Computer Operation
This chapter provides detailed information on operating the instrument from a
computer over either the GPIB (IEEE-488) or RS232 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.
Appendix C
Lists three simple terminal programs which may be used as the basis for more
complex user-written programs.
1-1
Chapter 1, INTRODUCTION
Appendix D
Shows the connection diagrams for suitable RS232 null-modem cables to couple the
unit to an IBM-PC or 100% compatible computer.
Appendix E
Provides a listing of the instrument settings produced by using the Auto-Default
functions.
Appendix F
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 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 F 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?
In its most basic form the lock-in amplifier is an instrument with dual capability. It
can recover signals in the presence of an overwhelming noise background or
alternatively it can provide high resolution measurements of relatively clean signals
over several orders of magnitude and frequency.
Modern instruments, such as the model 7280, offer far more than these two basic
characteristics and it is this increased capability which has led to their acceptance in
many fields of scientific research, such as optics, electrochemistry, materials science,
fundamental physics and electrical engineering, as units which can provide the
optimum solution to a large range of measurement problems.
The model 7280 lock-in amplifier can function as a:-
AC Signal Recovery Instrument Transient Recorder
Vector Voltmeter Precision Oscillator
Phase Meter Frequency Meter
Spectrum Analyzer Noise Measurement Unit
These characteristics, all available in a single compact unit, make it an invaluable
addition to any laboratory.
1-2
1.3 Key Specifications and Benefits
The SIGNAL RECOVERY Model 7280 represents a significant advance in the
application of DSP technology in the design of a lock-in amplifier. Until now,
limitations in the available semiconductor devices have restricted the operating
frequency range of such instruments to at most a few hundred kilohertz. The model
7280, with its use of the latest technology, extends this limit to 2 MHz. What is more,
it does this without compromising any other important specifications.
Key specifications include:
Frequency range: 0.5 Hz to 2.0 MHz
Voltage sensitivity: 10 nV to 1 V full-scale
Current input mode sensitivities: 1 pA to 100 µA full-scale
Line frequency rejection filter
Chapter 1, INTRODUCTION
10 fA to 1 µA full-scale
10 fA to 10 nA full-scale
Dual phase demodulator with X-Y and R-θ outputs
Very low phase noise of < 0.0001
Output time constant: 1 µs to 100 ks
5-digit output readings
Dual reference mode - allows simultaneous measurement of two signals at
different reference frequencies up to 20 kHz, or 800 kHz with option 7280/99 or
2.0 MHz with option 7280/98
Single and dual harmonic mode - allows simultaneous measurement of up to two
different harmonics of a signal at up to 20 kHz, or 800 kHz with option 7280/99
or 2.0 MHz with option 7280/98
Virtual reference mode - allows reference free measurement of signals up to
2.0 MHz
Spectral Display mode shows frequency spectrum of the signal prior to the
demodulators to help in selecting a reference frequency
Direct Digital Synthesizer (DDS) oscillator with variable amplitude and
frequency
°
rms
Oscillator frequency and amplitude sweep generator
8-bit programmable digital I/O port for external system control
Four auxiliary ADC inputs and two auxiliary DAC outputs
1-3
Chapter 1, INTRODUCTION
Full range of auto-modes
Non-volatile memory for 8 complete instrument settings
Standard IEEE-488 and RS232 interfaces with RS232 daisy-chain capability for
Large high-resolution electroluminescent display panel with menus for control
Easy entry of numerical control settings using keypad
32,768 point internal curve storage buffer
up to 16 instruments
and display of instrument outputs in both digital and graphical formats
1-4
Installation &
Initial Checks
2.1 Installation
2.1.01 Introduction
Installation of the model 7280 in the laboratory or on the production line is very
simple. It can be operated on almost any laboratory bench or be rack mounted, using
the optional accessory kit, 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 instrument uses forced-air ventilation and as such should be located so that the
ventilation holes on the sides and rear panels are not obstructed. This condition is
best satisfied by leaving a space of at least 2" (5 cm) between the side and rear panels
and any adjacent surface.
2.1.02 Rack Mounting
Chapter 2
An optional accessory kit, part number K02004, is available from
SIGNAL RECOVERY to allow the model 7280 to be mounted in a standard 19-inch
rack.
2.1.03 Inspection
Upon receipt the model 7280 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
A standard IEC 320 socket is mounted on the rear panel of the instrument and a
suitable line cord is supplied.
2.1.05 Line Voltage Selection and Line Fuses
Before plugging in the line cord, ensure that the model 7280 is set to the voltage of
the AC power supply to be used.
A detailed discussion of how to check and, if necessary, change the line voltage
setting follows.
CAUTION: The model 7280 may be damaged if the line voltage is set for 110 V AC
operation and it is turned on with 220 V AC applied to the power input connector.
The model 7280 can operate from any one of four different line voltage ranges, 90110 V, 110-130 V, 200-240 V, and 220-260 V, at 50-60 Hz. The change from one
range to another is made by repositioning a plug-in barrel selector internal to the Line
2-1
Chapter 2, INSTALLATION & INITIAL CHECKS
Input Assembly on the rear panel of the unit. Instruments are normally shipped from
the factory with the line voltage selector set to 110-130 V AC, unless they are
destined for an area known to use a line voltage in the 220-260 V range, in which
case, they are shipped configured for operation from the higher range.
The line voltage setting can be seen through a small rectangular window in the line
input assembly on the rear panel of the instrument (figure 2-1). If the number
showing is incorrect for the prevailing line voltage (refer to table 2-1), then the barrel
selector will need to be repositioned as follows.
Observing the instrument from the rear, note the plastic door immediately adjacent to
the line cord connector (figure 2-1) on the left-hand side of the instrument. When the
line cord is removed from the rear-panel connector, the plastic door can be opened
outwards by placing a small, flat-bladed screwdriver in the slot on the right-hand side
and levering gently. This gives access to the fuse and to the voltage barrel selector,
which is located at the right-hand edge of the fuse compartment. Remove the barrel
selector with the aid of a small screwdriver or similar tool. With the barrel selector
removed, four numbers become visible on it: 100, 120, 220, and 240, only one of
which is visible when the door is closed. Table 2-1 indicates the actual line voltage
range represented by each number. Position the barrel selector such that the required
number (see table 2-1) will be visible when the barrel selector is inserted and the door
closed.
Figure 2-1, Line Input Assembly
VISIBLE # VOLTAGE RANGE
100 90 - 110 V
120 110 - 130 V
220 200 - 240 V
240 220 - 260 V
Table 2-1, Range vs. Barrel Position
Next check the fuse rating. For operation from a nominal line voltage of 100 V or
120 V, use a 20 mm slow-blow fuse rated at 2.0 A, 250 V. For operation from a
nominal line voltage of 220 V or 240 V, use a 20 mm slow-blow fuse rated at 1.0 A,
250 V.
To change the fuse, first remove the fuse holder by pulling the plastic tab marked
with an arrow. Remove the fuse and replace with a slow-blow fuse of the correct
voltage and current rating. Install the fuse holder by sliding it into place, making sure
the arrow on the plastic tab is pointing downwards. When the proper fuse has been
installed, close the plastic door firmly. The correct selected voltage setting should
now be showing through the rectangular window. Ensure that only fuses with the
2-2
required current and voltage ratings and of the specified type are used for
replacement. The use of makeshift fuses and the short-circuiting of fuse holders is
prohibited and potentially dangerous.
2.2 Initial Checks
2.2.01 Introduction
The following procedure checks the performance of the model 7280. In general, this
procedure should be carried out after inspecting the instrument for obvious shipping
damage.
NOTE: Any damage must be reported to the carrier and to SIGNAL RECOVERY
immediately. In addition the shipping container must be retained for inspection by
the carrier.
Note that this procedure is intended to demonstrate that the instrument has arrived in
good working order, not that it meets specifications. Each instrument receives a
careful and thorough checkout before leaving the factory, and normally, if no
shipping damage has occurred, will perform within the limits of the quoted
specifications. If any problems are encountered in carrying out these checks, contact
SIGNAL RECOVERY or the nearest authorized representative for assistance.
Chapter 2, INSTALLATION & INITIAL CHECKS
2.2.02 Procedure
1) Ensure that the model 7280 is set to the line voltage of the power source to be
used, as described in section 2.1.05.
2) With the rear-panel mounted power switch (located to the right of the line power
input connector) set to 0 (off), plug in the line cord to an appropriate line source.
3) Turn the model 7280 power switch to the I (on) position.
4) The instrument's front panel display will now briefly display the following:-
2-3
Chapter 2, INSTALLATION & INITIAL CHECKS
5) Wait until the opening display has changed to the Main Display and then press the
key under the bottom right hand corner of the display identified by the legend
MENU on the display. This enters the first of the two main menus, Main Menu 1,
shown below in figure 2-3.
Figure 2-2, Opening Display
2-4
Figure 2-3, Main Menu 1
6) Press one of the keys adjacent to the Auto functions menu item to enter the Auto
Functions menu, shown below in figure 2-4.
Chapter 2, INSTALLATION & INITIAL CHECKS
Figure 2-4, Auto Functions Menu
7) Press one of the keys adjacent to the Auto Default menu item. This will set all of
the instrument's controls and the display to a defined state. The display will
revert to the Main Display, as shown below in figure 2-5, with the right-hand
side showing the vector magnitude, R, and the phase angle, θ, of the measured
signal in digital form, with two bar-graphs showing the X channel output and Y
channel output expressed in millivolts. The left-hand side shows five instrument
controls, these being the AC Gain in decibels, full-scale sensitivity, output time
constant, reference phase and internal oscillator frequency. The resulting
dynamic reserve (DR), in decibels, is also shown.
Figure 2-5, Main Display
8) Connect a BNC cable between the OSC OUT and A input connectors on the
front panel.
2-5
Chapter 2, INSTALLATION & INITIAL CHECKS
9) The right-hand side of the display should now indicate R, the vector magnitude,
close to 100% of full-scale (i.e. the sinusoidal oscillator output, which was set to
1 kHz and a signal level of 0.5 V rms by the Auto-Default function, is being
measured with a full-scale sensitivity of 500 mV rms) and θ, the phase angle, of
near zero degrees, if a short cable is used.
This completes the initial checks. Even though the procedure leaves many functions
untested, if the indicated results were obtained then the user can be reasonably sure
that the unit incurred no hidden damage in shipment and is in good working order.
2.3 Line Frequency Filter Adjustment
2.3.01 Introduction
The model 7280 incorporates a line-frequency rejection filter which is normally
supplied set to 60 Hz. If the power line frequency of the country in which the
instrument is to be used is also 60 Hz then the setting does not need to be changed. If,
however, the unit is to be used in an area with a 50 Hz power line frequency the
setting should be changed using the following procedure.
2.3.02 Procedure
1) Turn the model 7280’s power switch to the I (on) position.
2) The instrument's front panel display will now briefly display the following:-
Figure 2-6, Opening Display
2-6
3) Wait until the opening display has changed to the Main Display and then press
the key under the bottom right hand corner of the display identified by the legend
MENU on the display once. This enters the first of the two main menus, Main
Menu 1, shown below in figure 2-7.
Chapter 2, INSTALLATION & INITIAL CHECKS
Figure 2-7, Main Menu 1
4) Press one of the keys adjacent to the Configuration menu item to enter the
Configuration menu, shown below in figure 2-8.
Figure 2-8, Configuration Menu
5) The present line frequency setting is shown in reversed text under the
LINE FREQUENCY label and is either 50 or 60 Hz. In figure 2-8, the filter is set
to 60 Hz. If this setting does not match the prevailing line frequency, then press a
key adjacent to this item once to change it.
6) Press the key marked MAIN DISPLAY once to return to the Main Display.
This completes the procedure for adjusting the line frequency filter.
2-7
Chapter 2, INSTALLATION & INITIAL CHECKS
2-8
Technical Description
3.1 Introduction
The model 7280 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 7280 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
Chapter 3
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 7280
this is referred to as the single reference mode.
The dual reference mode incorporated in the model 7280 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, such as the requirement that one of the reference signals be external and
the other be derived from the internal oscillator, the limitation of the maximum
operating frequency to 20 kHz (unless the unit is equipped with the -/99 or -/98
extended frequency options) and the requirement that both signals be passed through
the same input signal channel. This last restriction 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.
3.2.03 Single Harmonic / Dual Harmonic
Normally, a lock-in amplifier measures the applied signal at the reference frequency.
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 7280 allows this multiple to be set
to any value between 2 (i.e. the second harmonic) and 32, as well as unity, which is
the normal mode. The only restriction is that the product n × f cannot exceed 2 MHz.
Dual harmonic mode allows the simultaneous measurement of two different
harmonics of the input signal. As with dual reference mode, there are a few
restrictions, such as a maximum value of n × f of 20 kHz (800 kHz with the option /99 or 2 MHz with the option -/98).
3-1
Chapter 3, TECHNICAL DESCRIPTION
3.2.04 Internal / External Reference Mode
In the internal reference mode, the instrument's reference frequency is derived from
its internal oscillator and the oscillator signal is used to drive the experiment.
In the external reference mode, the experiment includes some device, for example an
optical chopper, which generates a reference frequency that is applied to one of the
lock-in amplifier's external reference inputs. The instrument's reference channel
"locks" to this signal and uses it to measure the applied input signal.
3.2.05 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, believed to be unique to SIGNAL RECOVERY lockin 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 7280 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 instrument is shown in
figure 3-1. The sections that follow describe how each functional block operates and
the effect it has on the instrument's performance.
3-2
Chapter 3, TECHNICAL DESCRIPTION
Figure 3-1, Model 7280 - 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 input device. In current mode a choice
of three 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 kΩ 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 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
3-3
Chapter 3, TECHNICAL DESCRIPTION
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 kΩ resistor. When in the float mode, the presence of
this resistor substantially reduces the problems which often occur in low-level lock-in
amplifier measurements due to ground loops.
Input Coupling Mode, Fast/Slow
When the input coupling mode is set to Slow, the signal channel gain is essentially
flat down to 0.5 Hz, but recovery from input overload conditions may take a long
time. Conversely, when the coupling mode is set to Fast, recovery from overload is
much faster, but there will be a noticeable roll-off in magnitude response and
significant phase shifts at frequencies below typically 20 Hz.
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 kΩ) 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, normal or wide bandwidth conversion
settings, but it should be noted that if the best possible performance is required a
separate current preamplifier, such as the SIGNAL RECOVERY models 181 or
5182, should be considered.
3.3.03 Line Frequency Rejection Filter
Following the signal input amplifier there is an option to pass the signal through a
line frequency rejection filter, which is designed to give greater than 40 dB of
attenuation at the power line frequencies of 50 Hz or 60 Hz and their second
harmonics at 100 Hz and 120 Hz.
The filter uses two cascaded rejection stages with "notch" characteristics, allowing it
to be set to reject signals at frequencies equal to either of, or both of, the fundamental
and second harmonic of the line frequency.
Instruments are normally supplied with the line frequency filter set to 60 Hz with the
filter turned off. If the prevailing line frequency is 50 Hz then the filter frequency
should be set to this value using the control on the Configuration menu (see section
2.3).
3.3.04 AC Gain and Dynamic Reserve
3-4
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
Chapter 3, TECHNICAL DESCRIPTION
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.
AC Gain (dB) INPUT LIMIT (mV)
0 1600
6 800
14 320
20 160
26 80
34 32
40 16
46 8
54 3.2
60 1.6
66 0.8
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
3.3.12) 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 which 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 which 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
0.7DR ×=
LimitInput
ySensitivit Scale-Full
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
3-5
Chapter 3, TECHNICAL DESCRIPTION
reserve is often expressed in decibels, for which
Applying this formula to the model 7280 at the maximum value of INPUT LIMIT
(1.6 V) and the smallest available value of FULL-SCALE SENSITIVITY (10 nV),
gives a maximum available dynamic reserve of about 1 × 10
this magnitude are available from any DSP lock-in amplifier but are based only on
arithmetical identities and do not give any indication of how the instrument actually
performs. In fact, all current DSP lock-in amplifiers become too noisy and inaccurate
for most purposes at reserves of greater than about 100 dB.
For the benefit of users who prefer to have the AC Gain value expressed in decibels,
the model 7280 displays the current value of Dynamic Reserve (DR) in this form, on
the input full-scale sensitivity control, for values up to 100 dB. Above 100 dB the
legend changes to “DR>100”.
3.3.05 Anti-Aliasing Filter
Prior to the main analog-to-digital converter (ADC) the signal passes through an antialiasing filter to remove unwanted frequencies which would cause a spurious output
from the ADC as a result of the sampling process.
))ratio a log(DR(as20dB)DR(in ×=
8
or 160 dB. Figures of
Consider the situation when the lock-in amplifier is measuring a sinusoidal signal of
frequency f
Hz. In order to ensure correct operation of the instrument the output values
f
sampling
representing the f
Hz, which is sampled by the main ADC at a sampling frequency
signal
frequency must be uniquely generated by the signal to be
signal
measured, and not by any other process.
However, if the input to the ADC has, in addition, an unwanted sinusoidal signal
with frequency f
Hz, where f1 is greater than half the sampling frequency, then this
1
will appear in the output as a sampled-data sinusoid with frequency less than half the
sampling frequency, f
= |f1 - nf
alias
indistinguishable from the output generated when a genuine signal at frequency f
|, where n is an integer. This alias signal is
sampling
alias
is sampled. Hence if the frequency of the unwanted signal were such that the alias
signal frequency produced from it was close to, or equal to, that of the wanted signal
then it is clear that a spurious output would result.
For example, if the sampling frequency were 7.5 MHz then half the sampling
frequency would be 3.75 MHz. Assume for a moment that the instrument could
operate at reference frequencies up to 3.75 MHz and let it be measuring a signal of
40 kHz accompanied by an interfering signal of 3.7 MHz. The output of the ADC
would therefore include a sampled-data sinusoid of 40 kHz (the required signal) and,
applying the above formula, an alias signal of 0.05 MHz, or 50 kHz (i.e. |3.75 MHz -
3.7 MHz|). 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.
3-6
To overcome this problem the signal is fed through the anti-aliasing filter which
restricts the signal bandwidth to an upper frequency of 2.0 MHz The filter is a
conventional elliptic-type, low-pass, stage, giving the lowest possible noise
Chapter 3, TECHNICAL DESCRIPTION
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
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; 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
Following the anti-alias filter the signal passes to the main analog-to-digital converter
running at a sampling rate of 7.5 MHz. The output from the converter feeds the
demodulator circuitry, which uses DSP techniques to implement the digital
multipliers and the first stage of the output low-pass filtering for each of the X and Y
channels.
The ADC output also passes to the output processor to allow the power spectral
density of the input signal to be calculated using a discrete Fourier transform, which
in many ways is similar to a fast Fourier transform (FFT). The results of this
calculation are shown on the Spectral Display menu.
However, 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
The reference channel in the instrument is responsible for implementing the reference
trigger/phase-locked loop, digital phase-shifter and internal oscillator look-up table
functional blocks on the block diagram (see figure 3-1). The processor generates a
series of phase values, output at a rate of one every 133 ns, which are used to drive
the reference channel inputs of the demodulators.
In dual reference mode operation, an externally derived reference frequency is
connected to the external reference input and a second reference is derived from the
internal oscillator. The reference circuit generates new phase values for each
individual channel and sends these to the demodulators. Further discussion of dual
reference mode occurs in section 3.3.12.
In single harmonic mode, the reference circuit generates the phase values of a
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Chapter 3, TECHNICAL DESCRIPTION
waveform at the selected harmonic of the reference frequency. Dual harmonic mode
operates in a similar way to dual reference mode, but in this case the reference circuit
generates phase values for both of the selected harmonics of the reference frequency.
Dual harmonic mode may therefore be used with either internal or external
references.
External Reference Mode
In external reference mode the reference source may be applied to either a general
purpose input, designed to accept virtually any periodic waveform with a 50:50
mark-space ratio and of suitable amplitude, or to a TTL-logic level input. Following
the trigger buffering circuitry the reference signal is passed to a digital phase-locked
loop (PLL) implemented in the reference circuit. This measures the period of the
applied reference waveform and from this generates the phase values.
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
which 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.
A TTL logic signal at the present reference frequency is provided at the REF MON
connector on the rear panel.
3.3.08 Phase-Shifter
The reference circuit also implements a digital reference phase-shifter, allowing the
phase values being sent to the demodulator DSP to be adjusted to the required value.
If the reference input is a sinusoid applied to the REF IN socket, 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.
3-8
The circuits connected to the REF IN socket detect a positive-going crossing of the
mean value of the applied reference voltage. Therefore when the reference input is
not sinusoidal, its effective phase is the phase of a sinusoid with a positive-going zero
crossing at the same point in time, and accordingly the reference phase is defined
with respect to this waveform. Similarly, the effective phase of a reference input to
the TTL REF IN socket is that of a sinusoid with a positive-going zero crossing at
the same point in time.
Chapter 3, TECHNICAL DESCRIPTION
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
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.
3.3.09 Internal Oscillator - General
The model 7280, in common with many other lock-in amplifiers, incorporates an
internal oscillator which may be used to drive an experiment. However, unlike most
other instruments, the oscillator in the model 7280 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 500 mHz to
2.0 MHz, and is implemented using a dedicated direct digital synthesis circuit.
3.3.10 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 22.5 MHz.
3.3.11 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.
3-9
Chapter 3, TECHNICAL DESCRIPTION
It is important to note that this type of phase-locked loop, unlike a conventional edgetriggered 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. Naturally, this cannot be used as a
source for the measurement.
3.3.12 Demodulators
The essential operation of the demodulators is to multiply the digitized output of the
signal channel by data sequences, called the X and Y demodulation functions, and to
operate on the results with digital low-pass filters (the output filters). The
demodulation functions, which are derived by use of a look-up table from the phase
values supplied by the reference channel DSP, are sinusoids with a frequency equal
to an integer multiple, n × f, of the reference frequency f. The Y demodulation
function is the X demodulation function delayed by a quarter of a period. The integer
n is called the reference harmonic number and in normal lock-in amplifier operation
is set to unity. Throughout this chapter, the reference harmonic number is assumed to
be unity unless otherwise stated.
The outputs from the X channel and Y channel multipliers feed the first stage of the
X channel and Y channel output filters, implemented as Finite Impulse Response
(FIR) stages with selectable 6 or 12 dB/octave roll-off. The filtered X channel signal
drives a 16-bit DAC that, for short time-constant settings, generates the signal at the
instrument's CH 1 analog output connector. Both signals are combined by a fast
magnitude algorithm and a switch then allows either the filtered Y channel signal or
the magnitude signal, again when using short time-constant settings, to be passed to a
second 16-bit DAC to give the signal at the instrument's CH 2 analog output. The
significance of the magnitude output is discussed later in section 3.3.15.
In addition the X and Y channel signals are fed to further low-pass filters before
subsequent processing by the instrument’s host microprocessor.
The demodulator output is 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 a given full-scale sensitivity.
In dual reference and dual harmonic modes, the demodulators generate two sets of
outputs, one for each of the two references or harmonics, and includes two sets (four
channels) of initial output filtering. These outputs are passed to the host processor for
further processing and, when the time constant is less than 1 ms, the X
and X
1
2
outputs are also converted by fast DACs to analog signals that appear at the CH 1
and CH 2 analog output connectors.
3-10
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