Stanford Research Systems SR865 Operation Manual

Operation Manual

SR865

2 MHz DSP Lock-in Amplifier

Revision 1.25

Stanford Research Systems

Certification

Stanford Research Systems certifies that this product met its published specificat ions at
the time of shipment.

Warranty

This Stanford Research Systems product is warranted against defects in materials and
workmanship for a period of one (1) year from the date of shipment.

Service

For warranty service or repair, this product must be returned to a Stanford Research
Systems authorized service facility. Contact Stanford Research Systems or an authorized
representative before returning this product for repair.

Information in this document is subject to change without notice.
Copyright © Stanford Research Systems, Inc., 2015. All rights reserved.
Stanford Research Systems, Inc.
1290-C Reamwood Avenue
Sunnyvale, California 94089

www.thinkSRS.com

Printed in U.S.A. Document number 9-01707-903

SR865 DSP Lock-in Amplifier

Safety and Preparation For Use i

Safety and Preparation for Use

Warning

Dangerous voltages, capable of causing injury or death, are present
in this instrument. Use extreme caution whenever the instrument
covers are removed. Do not remove the covers while the unit is
plugged into a live outlet.

Line Voltage Selection

Caution

This instrument may be damaged if operated with the LINE VOLTAGE SELECTOR set
for the wrong ac line voltage or if the wrong fuse is installed.

The SR865 operates from a 100V, 120V, 220V, or 240V nominal ac power source having
a line frequency of 50 or 60 Hz. Before connecting the power cord, verify that the LINE
VOLTAGE SELECTOR card, located in the rear panel fuse holder, is set so that the
correct ac input voltage value is indicated by the white dot.

Conversion to other ac input voltages requires a change in the voltage selector card
position and fuse value. See Appendix F (page 177) for detailed instructions.

Line Fuse

Verify that the correct line fuse is installed before connecting the line cord. For
100V/120V, use a 1 Amp fuse and for 220V/240V, use a 1/2 Amp fuse. See Appendix F
(page 177) for detailed fuse installation instructions.

Line Cord

The SR865 has a detachable, three-wire power cord for connection to the power source
and to a protective ground. The exposed metal parts of the instrument are connected to
the outlet ground to protect against electrical shock. Always use an outlet which has a
properly connected protective ground. Power Cord

Grounding

A chassis grounding lug is available on the back panel of the SR865. Connect a heavy
duty ground wire, #12AWG or larger, from the CHASSIS GROUND lug directly to a
facility earth ground to provide additional protection against electrical shock.

Grounded BNC shields are connected to the chassis ground. Do not apply any voltage to
the grounded shields. The A and B signal input shields are connected to chassis ground
through resistors and can tolerate up to 1 V of applied voltage.

GFCI (Ground Fault Circuit Int errupter)

GFCI protected outlets are often available in production and laboratory environments,
particularly in proximity to water sources. GFCI’s are generally regarded as an important
defense against electrocution. However, the use of GFCI in conjunction with the SR865

SR865 DSP Lock-in Amplifier

ii Safety and Preparation For Use

must not be regarded as a substitute for proper grounding and careful system design.
GFCI’s must also be tested regularly to verify their functionality. Always consult an
electrician when in doubt.

Service

Do not attempt to service this instrument unless another person, capable of providing first
aid or resuscitation, is present.

Do not install substitute parts or perform any unauthorized modifications to this
instrument. Contact the factory for instructions on how to return the instrument for
authorized service and adjustment.

Warning Regarding Use Wi t h Photom ultipliers and Other
Detectors

The front end amplifier of this instrument is easily damaged if a photomultiplier is used
improperly with the amplifier. When left completely unterminated, a cable connected to a
PMT can charge to several hundred volts in a relatively short time. If this cable is
connected to the inputs of the SR865 the stored charge may damage the front-end
amplifier. To avoid this problem, always discharge the cable and connect the PMT output
to the SR865 input before turning the PMT on.

Furnished Accessories

• Power Cord

• Operating Manual

Environmental Conditi ons

Operating

Temperature: +10 °C to +40 °C
(Specifications apply over +18 °C to +28 °C)
Relative Humidity: <90 % Non-condensing

Non-Operating

Temperature: −25°C to 65°C
Humidity: <95 % Non-condensing

SR865 DSP Lock-in Amplifier

Safety and Preparation For Use iii

SR865 DSP Lock-in Amplifier

iv Safety and Preparation For Use

SR865 DSP Lock-in Amplifier

Contents v

Contents

Safety and Preparation for Use i
Contents v
SR865 Specifications vii
SR865 Command List x
SR865 Status Bytes xv

Chapter 1 Getting Started 1

Introduction 1
SR865 Front Panel 2
SR865 Touchscreen 4
The Basic Lock-in 9
Using Displays 13
Sensitivity, Offset and Expand 20
Saving and Recalling Setups 25
Aux Outputs and Inputs 28
Scanning 30

Chapter 2 Lock-in Amplifier Basics 37

What is a Lock-in Amplifier? 37
What Does a Lock-in Measure? 40
Block diagram 41
The Reference Oscillator 42
The Phase Sensitive Detectors 43
Time Constants and Sensitivity 44
Outputs and Scales 46
What is Dynamic Reserve Really? 48
The Input Amplifier 50
Input Connections 51
Intrinsic (Random) Noise Sources 54
External Noise Sources 55
Noise Measurements 58

Chapter 3 Operation 61

Introduction 61
Standard Settings 64
Signal Input 65
CH1 and CH2 Outputs: Offset, Ratio and Expand 71
Reference 74
Display 81
Cursor 83

SR865 DSP Lock-in Amplifier

vi Contents

Functions 85
Interface and Data 87
Setup 88
Rear Panel 96

Chapter 4 Programming 101

Introduction 101
Command Syntax 103
Reference Commands 106
Signal Commands 111
CH1/CH2 Output Commands 114
Aux Input and Output Commands 116
Auto Function Commands 117
Display Commands 118
Strip Chart Commands 120
FFT Screen Commands 125
Scan Commands 128
Data Transfer Commands 132
Data Capture Commands 134
Data Streaming Commands 140
System Commands 143
Interface Commands 146
Status Reporting Commands 148
Status Byte Definitions 151

Appendix A Advanced Filters 155
Appendix B The FFT Display 161
Appendix C Using the Webserver 169
Appendix D Data Streaming and Capture 171
Appendix E Dual Reference Detection 175
Appendix F Fuse Installation and ac Line Select 177
Appendix G Performance Tests 181
Appendix H Circuit Description 201

SR865 DSP Lock-in Amplifier

Specifications vii

SR865 Specifications

Signal Channel

Voltage Inputs Single-ended (A) or differential (A−B)
Sensitivity (Output Scale) 1 nV to 1 V (voltage input)
1 fA to 1 µA (current input)
Input Impedance 10 MΩ+25 pF, ac (>1 Hz) or dc coupled
Input Range 10 mV to 1 V (peak); max input before overload
Gain Accuracy 1% below 200 kHz and 2% to 2 MHz (signal amplitude <30% of input range)
Input Noise 2.5 nV/√Hz at 1 kHz, 10 mV input range (typical)
CMRR Greater than 90 dB at 1 kHz (dc Coupled)
Harmonic Distortion −80 dB below 100 kHz, −60 dB above 100 kHz
Dynamic Reserve Greater than 120 dB
Current Input Ranges 1 μA or 10 nA

Reference Channel

Frequency Range 1 mHz to 2 MHz specified (operates to 2.5 MHz)
Timebase 10 MHz In/Out phase locks the internal frequency to other SR865 units
Ext TTL Reference Minimum 2 V logic level, rising or falling edge
Ext Sine Reference 400 mV pk–pk minimum signal, ac coupled (>1 Hz)
Ext Reference Input Impedance 1 MΩ or 50 Ω
Acquisition Time (2 cycles + 5 ms) or 40 ms, whichever is greater
Phase Setting Resolution 360/2
Phase Noise Ext TTL reference: <0.001° rms at 1 kHz, (100 ms, 12 dB/oct) (typical)
Internal reference: <0.0001° rms at 1 kHz (100 ms, 12 dB/oct)
Phase Drift Sine Out to Signal In (200 mVrms)
<0.002°/°C below 20 kHz (dc coupled input)
<0.02°/°C below 200 kHz
<0.2°/°C below 2 MHz
Harmonic Detect Detect at N×f
Dual F Reference Detect at f
All frequencies less than 2 MHz for specified performance (operates to 2.5 MHz)
Chopper Reference SR865 drives SR540 Chopper (via Aux Out 4) to lock the chopper to f

32

deg

where N≤99 and N×f

ref

= | f

− f

dual

|

int

ext

<2 MHz

ref

int

Demodulator

dc Stability Digital output values have no offset drift
Time Constants 1 µs to 30 ks
Low Pass Filters Typical RC type filters or Advanced Gaussian/Linear Phase filters
Filter Slope 6, 12, 18, 24 dB/oct rolloffs
Synchronous Filter Available below 4 kHz
Harmonic Rejection −80 dB
Low Latency Output Rear panel BlazeX output with <2 µs delay (plus low pass filter rise/fall times)

Internal Oscillator

Frequency 1 mHz to 2 MHz specified (operates to 2.5 MHz)
Frequency Accuracy 25 ppm + 30 µHz with internal timebase
External Timebase 10 MHz timebase input/output on rear panel
Frequency Resolution 6 digits or 0.1 mHz, whichever is greater

SR865 DSP Lock-in Amplifier

viii Specifications

Sine Output

Outputs Differential or Single-ended
Output Impedance 50 Ω source
Amplitude 1 nVrms to 2 Vrms (specified amplitude is differential into 50 Ω loads)
Output amplitude is halved when used single-ended
Output amplitude is doubled into a high impedance load
Amplitude Resolution 3 digits or 1 nV, whichever is greater
dc Offset ±5 V, differential or common mode
Offset Resolution 3 digits or 0.1 mV, whichever is greater
Output Limit ±6 V, sum of dc offset and peak amplitude
Sync Logic level sync on rear panel (via BlazeX output)

Data

Data Channels 4 data channels are displayed and graphed (green, blue, yellow, orange)
Data Sources Each data channel can be assigned any of these data sources:

, Y

X, Y, R, θ, Aux In 1–4, Aux Out 1–2, X
DC Level, reference phase, f

int

or f

noise

ext

Data History A ll data sources are continuo u sly stored at all chart display time scales.

The complete stored history of any data source can be displayed at any time.
Offset X, Y and R may be offset up to ±999% of the sensitivity
Ratio X, and Y may be ratioed by Aux In 3; R may be ratioed by Aux In 4
Expand X, Y and R may be expanded by ×10 or ×100
Capture Buffer 1 Mpoints internal data storage. Store (X), (X and Y), (R and θ) or (X, Y, R and θ)

at sample rates up to 1.25 MHz. This is in addition to the data histories for the

chart display.
Data Streaming Realtime streaming of data, either (X), (X and Y), (R and θ) or (X, Y, R and θ) at

sample rates up to 1.25 MHz over Ethernet interface
Scanning One of the following parameters may be scanned:

f

, Sine Out Amplitude, Sine Out DC Level, Aux Out 1 or 2.

int

, Sine Out Amplitude, Sine Out

noise

FFT

Source Input ADC, demodulator output, or filter output
Record length 1024 bins
Averaging exponential rms

Inputs and Outputs

CH 1 Output Proportional to X or R, ±10 V full scale thru 50 Ω
CH 2 Output Proportional to Y or θ, ±10 V full scale thru 50 Ω
X and Y Outputs Proportional to X and Y, ±10 V full scale thru 50 Ω, rear panel
BlazeX Low latency output of X, ±2.0 V full scale or

logic level reference sync output, either thru 50 Ω
Aux Outputs 4 BNC D/A outputs, ±10.5 V thru 50 Ω, 1 mV resolution
Aux Inputs 4 BNC A/D inputs, ±10.5 V, 1 mV resolution, 1 MΩ input
Trigger Input TTL input triggers storage into the internal capture buffer
Monitor Output Analog output of the signal amplifier
HDMI Video output to external monitor or TV, 640x480/60 Hz.
Timebase Input/Output 1 Vrms 10 MHz clock to synchronize internal reference frequency to other units

General

Interfaces IEEE488, RS-232, USB device (Test and Measurement Class) and Ethernet

(VXI-11 and telnet)

SR865 DSP Lock-in Amplifier

Specifications ix

USB Flash Front panel slot for USB flash storage of screen shots and data, and firmware

upgrades
Preamp Power 9 pin D connector to power SRS preamps
Power 60 Watts, 100/120/220/240 VAC, 50/60 Hz
Dimensions 17"W × 5.25"H × 17"D
Weight 22 lbs
Warranty One year parts and labor on materials and workmanship

SR865 DSP Lock-in Amplifier

x Commands

Reference Commands

page

description

TBMODE(?) { AUTO | INternal | i }

106

Set the 10 MHz timebase

TBSTAT?

106

Query the current 10 MHz timebase ext (0) or int (1)

PHAS(?) { p } { UDEG | MDEG | DEG | URAD | MRAD | RAD }

106

Set the reference phase to p

APHS

106

Auto Phase

FREQ(?) { f } { HZ | KHZ | MHZ }

106

Set the reference frequency to f

FREQINT(?) { f } { HZ | KHZ | MHZ }

107

Set the internal reference frequency to f

FREQEXT?

107

Query the external reference frequency

FREQDET?

107

Query the detection frequency

HARM(?) { i }

107

Set harmonic detect to i

HARMDUAL(?) { i }

107

Set harmonic for dual reference mode to i

BLADESLOTS(?) {SLT6 | SLT30 | i }

108

Set the chopper blade number of slots

BLADEPHASE(?) { p } { UDEG | MDEG | DEG | URAD | MRAD |
RAD }

108

Set the chopper blade phase to p
SLVL(?) { v } { NV | UV | MV | V }

108

Set sine output amplitude to v

SOFF(?) { v } { NV | UV | MV | V }

108

Set sine output dc level to v

REFM(?) { COMmon | DIFference | i }

109

Set sine output dc mode

RSRC(?) { INT | EXT | DUAL | CHOP | i }

109

Set reference mode

RTRG(?) { SIN | POSttl | NEGttl | i }

109

Set external reference trigger

REFZ(?) { 50ohms | 1Meg | i }

109

Set external reference input impedance

PSTF(?) [ j ] { , f { HZ | KHZ | MHZ } }

110

Set frequency preset j to f

PSTA(?) [ j ] { , v { NV | UV | MV | V } }

110

Set sine amplitude preset j to v

PSTL(?) [ j ] { , v { NV | UV | MV | V } }

110

Set sine dc level preset j to v

Signal Commands

page

description

IVMD(?) { VOLTage | CURRent | i }

111

Set input to voltage/current

ISRC(?) { A | A−B | i }

111

Set voltage input configuration

ICPL(?) { AC | DC | i }

111

Set voltage input coupling

IGND(?) { FLOat | GROund | i }

111

Set voltage input shield grounding

IRNG(?) { 1Volt | 300Mvolt | 100Mvolt | 30Mvolt | 10Mvolt | i }

111

Set voltage input range

ICUR(?) { 1MEG | 100MEG | i }

112

Set current input gain

ILVL?

112

Query the signal strength low to overload (0–4)

SCAL(?) { i }

112

Set sensitivity from 1V (0) to 1 nV (27)

OFLT(?) { i }

113

Set time constant from 1 μs (0) to 30 ks (21)

OFSL(?) { i }

113

Set filter slope from 6 dB (0) to 24 dB (3)

SYNC(?) { OFF | ON | i }

113

Turn synchronous filter off/on

ADVFILT(?) { OFF | ON | i }

113

Turn advanced filtering off/on

ENBW?

113

Query the equivalent noise bandwidth

CH1/CH2 Output Commands

page

description

COUT(?) [ OCH1 | OCH2 | j ] { , XY | RTHeta | i }

114

Set CH1/2 to (X or R)/(Y or θ)

SR865 Command List

SR865 DSP Lock-in Amplifier

Commands xi

CEXP(?) [ X | Y | R | j ] { , OFF | X10 | X100 | i }

114

Set CH1/2 expand

COFA(?) [ X | Y | R | j ] { , OFF | ON | i }

114

Turn CH1/2 output offset off/on

COFP(?) [ X | Y | R | j ] { , x }

114

Set CH1/2 output offset to x percent

OAUT [ X | Y | R | j ]

114

Auto Offset CH1/2

CRAT(?) [ X | Y | R | j ] { , OFF | ON | i }

115

Ratio Ch1/2 with Aux In 3 or 4

Aux Input and Output Commands

page

description

OAUX? [ j ]

116

Query Aux Input j (0–3)

AUXV(?) [ j ] { , v { NV | UV | MV | V } }

116

Set Aux Output j (0–3) voltage to v

Auto Function Commands

page

description

APHS

117

Auto Phase

ARNG

117

Auto Range

ASCL

117

Auto Scale

Display Commands

page

description

DBLK(?) { OFF | ON | i }

118

Turn front panel blanking off/on

DLAY(?) { TREnd | HISTory | BARHist | FFT | BARFft | BAREight | i }

118

Set Screen Layout

DCAP

118

Screen Shot to USB memory stick

CDSP(?) [ DAT1 | DAT2 | DAT3 | DAT4 | j ] { , parameter | i }

118

Assign parameter i to data channel j

CGRF(?) [ DAT1 | DAT2 | DAT3 | DAT4 | j ] { , OFF | ON | i }

119

Turn data channel j strip graph off/on

GETSCREEN?

119

Download screen capture image

Strip Chart Commands

page

description

GSPD(?) { i }

120

Set horizontal scale from 0.5 s (0) to 2 days (16)

GSCL(?) [ DAT1 | DAT2 | DAT3 | DAT4 | j ] { , x }

120

Set channel j vertical scale to x

GOFF(?) [ DAT1 | DAT2 | DAT3 | DAT4 | j ] { , x }

120

Set channel j vertical offset

GAUT [ DAT1 | DAT2 | DAT3 | DAT4 | j ]

121

Auto scale channel j

GACT [ DAT1 | DAT2 | DAT3 | DAT4 | j ]

121

Auto scale with zero center channel j

GAUF [ DAT1 | DAT2 | DAT3 | DAT4 | j ]

121

Auto find for channel j

CGRF(?) [ DAT1 | DAT2 | DAT3 | DAT4 | j ] { , { OFF | ON | i } }

121

Turn channel j graph off/on

GLIV(?) { OFF | ON | i }

122

Turn strip chart off/on (pause/run)

PCUR(?) { i }

122

Move cursor to i (0=right, 639=left)

CURREL(?) {OFF | ON | i}

122

Set cursor relative or absolute mode

CURDISP(?) {i}

122

Set cursor horizontal mode to date/time or interval

CURBUG(?) {AVG | MAX | MIN | i}

122

Set cursor to mean / maximum / minimum

FCRW(?) { LIne | NARrow | WIde | i }

123

Set the cursor width

SCRY? [ DAT1 | DAT2 | DAT3 | DAT4 | STATus | j ]

123

Query the cursor data values

CURDATTIM?

123

Query the cursor horizontal position as date/time

CURINTERVAL?

124

Query the cursor horizontal position as interval

FFT Screen Commands

page

description

SR865 DSP Lock-in Amplifier

xii Commands

FFTR(?) { ADC | MIXer | FILTer | i }

125

Set FFT data source

FFTS(?) { i }

125

Set vertical scale from 1.0 dB (0) to 200 dB (7)

FFTO(?) { x }

125

Set top reference level

FAUT

126

FFT auto scale

FFTMAXSPAN?

126

Query maximum allowed frequency span in Hz/div

FFTSPAN(?) { x }

126

Set frequency span in Hz/div

FFTA(?) { AVG1 | AVG3 | AVG1 0 | AVG30 | AVG100 | i }

126

Set averaging

FFTL(?) { OFF | ON | i }

126

Turn FFT off/on (pause/live)

FCRW(?) { LIne | NARrow | WIde | i }

126

Set the cursor width

FCRX?

127

Query the FFT cursor frequency

FCRY?

127

Query the FFT cursor amplitude

Scan Commands

page

description

SCNPAR(?) { Fint | REFAmp | REFDc | OUT1 | OUT2 | i }

128

Set the scan parameter

SCNLOG(?) { LIN | LOG | i }

128

Set the scan type

SCNEND(?) { ONce | REpeat | UPdown | i }

128

Set the scan end mode

SCNSEC(?) { x }

128

Set the scan time to x seconds

SCNAMPATTN(?) {i}

128

Set the amplitude attenuator mode for scanning

SCNDCATTN(?) {i}

129

Set the dc attenuator mode for scanning

SCNINRVL(?) {i}

129

Set the scan parameter update interval

SCNENBL(?) { OFF | ON | i }

129

Turn the scan off/on

SCNRUN

130

Start or resume the scan

SCNPAUSE

130

Pause the scan

SCNRST

130

Reset the scan

SCNSTATE?

130

Query the scan (off, reset, run, pause or done)(0-4)

SCNFREQ(?) [ BEGin | END | j ] { , f { HZ | KHZ | MHZ } }

130

Set the begin/end frequency

SCNAMP(?) [ BEGin | END | j ] { , v { NV | UV | MV | V } }

130

Set the begin/end reference amplitude

SCNDC(?) [ BEGin | END | j ] { , v { NV | UV | MV | V } }

130

Set the begin/end reference dc level

SCNAUX1(?) [ BEGin | END | j ] { , v { NV | UV | MV | V } }

131

Set the begin/end AuxOut1 value

SCNAUX2(?) [ BEGin | END | j ] { , v { NV | UV | MV | V } }

131

Set the begin/end AuxOut2 value

Data Transfer Commands

page

description

OUTR? [ DAT1 | DAT2 | DAT3 | DAT4 | j ]

132

Query data channel j

OUTP? [ j ]

132

Query lock-in parameter j

SNAP? [ j, k ] { , l }

132

Query multiple lock-in parameters at once

Data Capture Commands

page

description

CAPTURELEN(?) { n }

136

Set the buffer length to n 1 kbyte blocks

CAPTURECFG(?) { X | XY | RT | XYRT | i }

136

Configure capture to X, XY, Rθ or XYRθ (0–3)

CAPTURERATEMAX?

136

Query the maximum capture rate

CAPTURERATE(?) { n }

136

Set the capture rate to (max rate)/2n

CAPTURESTART [ ONEshot | CO NTinuous | i ] , [ OFF | ON | j ]

137

Start capture (OneShot or Cont) (HW trigger off/on)

CAPTURESTOP

137

Stop capture

SR865 DSP Lock-in Amplifier

Commands xiii

CAPTURESTAT?

137

Query the capture buffer state

CAPTUREPROG?

138

Query the length (kB) of captured data (after stop)

CAPTUREVAL? [ n ]

138

Query the nth sample (1, 2 or 4 values) (after stop)

CAPTUREGET? [ i ], [ j ]

138

Download binary capture buffer

Data Streaming Commands

page

description

STREAMCH(?) { X | XY | RT | XYRT | i }

140

Configure streaming to X, XY, Rθ or XYRθ (0–3)

STREAMRATEMAX?

141

Query the maximum streaming rate

STREAMRATE(?) { n }

141

Set the streaming rate to (max rate)/2n

STREAMFMT(?) { i }

141

Set the streaming format to float32 (0) or int16 (1)

STREAMPCKT(?) { i }

141

Set packet size to 1024, 512, 256 or 128 bytes (0–3)

STREAMPORT(?) { i }

142

Sets the Ethernet port to i=1024–65535

STREAMOPTION(?) { i }

142

Sets big/little endianness and integrity checking

STREAM(?) { OFF | ON | i }

142

Turn streaming off/on

System Commands

page description

TIME(?) [ SEConds | MINutes | HOUrs | j ] { , i }

143

Set time

DATE(?) [ DAY | MONth | YEAr | j ] { , i }

143

Set date

TBMODE(?) { AUTO | INternal | i }

143

Set the 10 MHz timebase

TBSTAT?

143

Query the current 10 MHz timebase ext (0) or int (1)

BLAZEX(?) { BLazex | BIsync | UNIsync | i }

143

Select the BlazeX output

KEYC(?) { ON | MUte| i }

144

Turn sounds on or off

PRMD(?) { SCReen | PRNt | MONOchrome | i }

144

Set screen shot mode

SDFM(?) { CSV | MATfile | i }

144

Set data file type

FBAS(?) { s }

144

Set file name prefix

FNUM(?) { i }

144

Set file name suffix

FNXT?

145

Query next file name

DCAP

145

Screen shot

SVDT

145

Save data

Interface Commands

page description

RST

146

Reset the unit to its default configuration

IDN?

146

Query the unit identification string

TST?

146

No-op, returns “0”

*OPC(?)

146

Operation Complete

LOCL(?) { i }

146

Set LOCAL (0), REMOTE (1) or LOCKOUT (2)

OVRM (?) { OFF | ON | i }

147

Set GPIB Overide Remote off (0) or on (1)

Status Reporting Commands

page

description

CLS

148

Clear all status bytes

ESE(?) { j, } { i }

148

Set the standard event enable register

*

*

*

*

*

SR865 DSP Lock-in Amplifier

xiv Commands

ESR? { j }

148

Query the standard event status byte

148

Set the serial poll enable register

STB? { j }

148

Query the serial poll status byte

PSC(?) { i }

149

Set the Power-On Status Clear bit

ERRE(?) { j, } { i }

149

Set the error status enable register

ERRS? { j }

149

Query the error status byte

LIAE(?) { j, } { i }

149

Set LIA status enable register

LIAS? { j }

150

Query the LIA status word

CUROVLDSTAT?

150

Query the present overload states

*

*SRE(?) { j, } { i }

*

*

SR865 DSP Lock-in Amplifier

Commands xv

Serial Poll Status Byte

bit

name

usage

0

unused

1

unused

2 ERR

An enabled bit in the error status byte has been set

3 LIA

An enabled bit in the LIA status byte has been set

4 MAV

The interface output buffer is non-empty

5 ESB

An enabled bit in the standard status byte has been set

6 SRQ

SRQ (service request) has occurred

7

unused

Standard Event Status Byte

bit

name

usage

0 OPC

Operation complete

1 INP

Input queue overflow

2

unused

3 QRY

Output queue overflow

4 EXE

A command cannot execute correctly or a parameter is out of range

5 CMD

An illegal command is received

6 URQ

Set by any user front panel action

7 PON

Set by power-on

LIA Status Word

bit

name

usage

0 CH1OV

CH1 output overload

1 CH2OV

CH2 output overload

2

unused

3 UNLK

External reference or Chop unlock detected

4 RANGE

Input range overload detected

5 SYNCF

Sync filter frequency out of range

6 SYNCOV

Sync filter overload

7 TRIG

Set when data storage is triggered

8 DAT1OV

Data Channel 1 output overload

9 DAT2OV

Data Channel 2 output overload

10

DAT3OV

Data Channel 3 output overload

11

DAT4OV

Data Channel 4 output overload

12

DCAPFIN

Display capture to USB stick completed

13

SCNST

Scan started

14

SCNFIN

Scan completed

SR865 Status Bytes

SR865 DSP Lock-in Amplifier

xvi Commands

Error Status Byte

bit

name

usage

0 CLK

External 10 MHz clock input error

1 BACKUP

Battery backup failed

2

unused

3

unused

4 VXI

VXI-11 error

5 GPIB

GPIB fast data transfer mode aborted

6 USBDEV

USB device error (interface error)

7 USBHOST

USB host error (memory stick error)

SR865 DSP Lock-in Amplifier

Getting Started 1

Chapter 1

Getting Started

Introduction

The sample measurements described in this section are designed to acquaint the first time
user with the SR865 DSP Lock-In Amplifier. Do not be concerned that your
measurements do not exactly agree with these exercises. The focus of these measurement
exercises is to learn how to use the instrument. It is highly recommended that the first
time user step through some or all of these exercises before attempting to perform an
actual experiment.

Keys, Knobs and Touch Buttons

[Key] Front panel keys are referred to in [square] brackets. Some keys have a

second italicized label. Press and hold these keys for 2 seconds to invoke

the italicized function.

<Knob> Knobs are referred to in <angle> brackets. Knobs are used to adjust

parameters which have a wide range of values. Some knobs have a push

button function. Some also have a second italicized label. Press and hold

these knobs for 2 seconds to invoke the italicized function.

{Touch} Touchscreen buttons and icons are referred to in {curly} brackets.

Touchscreen buttons are used to adjust the data display as well as change

certain lock-in parameters.

SR865 DSP Lock-in Amplifier

2 Getting Started Chapter 1

SR865 Front Panel

The SR865’s buttons and knobs are mainly used to configure the lock-in measurement,
while the touchscreen is mainly for data display. The touchscreen display is also used for
keypad entry.

Signal Path

The signal path settings configure input BNC’s, input gain, time constant and filters and
the sensitivity. Unlike previous generation lock-ins, the SR865 does not have a dynamic
reserve setting. The input range setting is simply the largest input signal before overload.
It is best to decrease the input range setting as much as possible without overload. This
increases the gain and utilizes more of the A/D converter’s range as indicated by the
signal strength LEDs.

The sensitivity determines the scale factor for the analog outputs (CH1 and CH2) as well
as the numeric readouts and bar graphs. The sensitivity does not affect the measurem ent
values, it simply determines how much signal corresponds to a full scale 10V output from
CH1 and CH2 outputs and a 100% bar graph. It also sets the scale for the 5 digit numeric
displays. The sensitivity should be viewed as an output function only.

Reference

The reference settings configure the lock-in reference frequency and source. In addition
to internal and external reference, the SR865 includes dual reference (detect at |f
and chop (lock an SR540 chopper TO the SR865 f

) modes.

int

The SR865 can be synchronized to an external 10 MHz frequency refere nce (f ro m
another SR865 or other source). This allows multiple SR865’s to run in phase sync with
each other in internal reference mode.

int

− f

ext

|)

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 3

The sine output from the SR865 is differential. This provides improved performance at
low amplitudes. A variable dc offset is provided in both differential and common mode.
Use either sine out for single ended excitation.

Outputs

The CH1 output can be proportional to either X or R, while the CH2 output can be
proportional to Y or θ. Output functions include offset (up to ±999% of the sensitivity),
expand (up to ×100) and ratio. These functions are generally only used when the CH1 or
CH2 outputs are being used to drive other parts of an experiment.

SR865 DSP Lock-in Amplifier

4 Getting Started Chapter 1

SR865 Touchscreen

The SR865 screen displays the lock-in outputs both numerically and graphically. Touch
buttons and icons are used to adjust the data displays as well as enter certain lock-in
parameters.

Screen Layout

Press [Screen Layout] to cycle through the different screen layouts.

Trend Graph Full Screen Strip Chart

Half Screen Strip Chart Full Screen FFT

Half Screen FFT Big Numbers

The SR865 displays up to 4 channels at a time, in green, blue, yellow and orange. Each
channel is assigned a parameter using the [Config] key. Parameters are chosen from X,

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 5

Y, R, θ (detected), f

, f

, phase (setting), Sine Amplitude, DC Level, any Aux Input,

int

ext

Aux Output 1 or 2, Xnoise, or Ynoise.

Displayed parameters can be re-assigned at any time. Data is being stored for all possible
parameters all of the time.

Info Bar and Numeric Entry

Each of the data screens always displays a lock-in info bar across the top.

This bar always shows tiles displaying the phase, frequency, detect harmonic, sine out
amplitude and dc offset of the sine out. Each of these parameters can be adjusted using
the knobs and buttons in the reference settings section of the front panel.

Touching one of these tiles brings up a numeric keypad for direct entry.

Numeric entry is straightforward. {Close} will return to the data screen. The buttons
{F1}, {F2}, {F3} and {F4} are frequency presets. Touching a preset will load the preset
value immediately. Touch and hold a preset button to memorize the current setting. Other
parameters may have slightly different entry screens.

Strip Charts

The most common way to visualize the lock-in outputs is to use the strip chart display.
New data is plotted at the right edge and older data scrolls left. The scrol l rate is
determined by the horizontal scale (time per division). For example, a scale of 1s/div
presents the 10 most recent seconds of data and data points take 10 s to scroll completely
off the left edge. Horizontal scales range from 0.5 s to 2 days per division.

At each point along the horizontal axis, the graph displays the maximum to minimum
excursion of each data channel during a time interval corresponding to that point in the

SR865 DSP Lock-in Amplifier

Internal frequency entry screen

6 Getting Started Chapter 1

past. The time interval is determined by the horizontal scale per division and the number
of pixels in the display. There are 640 pixels across 10 divisions of the graph. Thus there
are 64 pixels in each division. At a scale of 0.5 s/div, each pixel represents about 8 ms of
data. At a scale of 1 min/div, each pixel represents about 1s of data. This ‘binning’ is
fundamental to the SR865 strip chart display. All time scales are stored all of the time.
This allows the horizontal scale to change without re-acquiring any data. The caveat is
that all graphs are drawn with the most recent point at the right hand edge.

Zooming in and out (changing the horizontal scale) always displays the most recent point
at the right edge. There is no zooming in about a point in the distant past.

All parameters which may be assigned to a data channel are continuously recorded even
when they are not displayed. This means that historical data can be viewed for all
parameters simply by assigning them to a data channel and viewing the strip chart.

Strip charts may be paused. When the graph is paused, the cursor can be used to readout
data values. Data storage continues in the background while the graph is paused. When
live scrolling is resumed, the graph is redrawn so the most recent point is once again at
the right edge.

Graph Scale Bar

Strip Chart displays have a scale bar at the bottom of the screen.

This bar shows tiles indicating the vertical scale per division for the 4 data channels
(green, blue, yellow and orange) and the horizontal time scale per division (white).

Touch a data channel’s scale tile to display a palette of scale functions.

Chart vertical scale palette

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 7

Use the palette functions to scale the selected data channel’s graph. Touch the scale tile
again to dismiss the palette.

Vertical Scale Palette Horizontal Scale Palette

Vertical Scale Palette

Vertical scale changes are applied to each data channel separately. {Autoscale} adjusts
the scale and center so the graph occupies as much of the screen as possible. {Autoscale
Zero Center} forces the center of the graph to be zero and then sets the scale to show the
data. The location of zero is indicated by the small triangle on the right edge. It points left
where zero is. It points up or down if zero is above or below the graph.

{Zoom In} and {Zoom Out} change the scale about the center. Use {Center Newest
Point} to bring the current point to the center of the graph before zooming in or out.
{Move Up} and {Move Down} simply move the graph up and down on the screen. The
graph can also be moved simply by touching and dragging on the screen while the
vertical scale palette is displayed.

Each graph can also be turned off. Touch the scale tile to turn the graph back on.

All changes to the graphs are non-destructive. They simply change the way data is
visualized. Stored parameter values are not altered by scale changes.

Horizontal Scale Palette

Horizontal scale changes are applied to the entire strip chart display and all data channels.

{Zoom In} and {Zoom Out} change the horizontal scale and scroll speed.

{Pause} stops the chart scrolling and pauses the graph. When the graph is paused, the
cursor can be used to readout data values. These readouts correspond to the min, max or
mean of the data in the time bin at the cursor location. The time of the cursor location is
displayed in the tile at the left edge of the scale bar below the graph. Touch this tile to
switch between elapsed time from the right edge to absolute time (time and date when the
point was taken). Use {Cursor MinMaxMean} and {Cursor Width} to change the cursor.
Note that the cursor marker may not lie on the data graph for wide cursors since the
marker shows the min, max or mean of all the data within the cursor width.

SR865 DSP Lock-in Amplifier

8 Getting Started Chapter 1

The cursor is only displayed when the graph is paused.

Zooming in and out preserves the right hand edge of the graph at the point in time when
the graph was paused.

Data storage continues in the background while the graph is paused. When live scrolling
is resumed with {Resume}, the graph is redrawn so the current point is once again at the
right edge.

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 9

Do This

Explanation

default settings. See the Standard Settings list in

the settings.

should set the values of Y and θ to zero.

The Basic Lock-in

This measurement is designed to use the internal oscillator to explore some of the basic lock-in functions.
Specifically, you will measure the amplitude of the Sine Out at various frequencies, amplitudes, time
constants and phase shifts.

1. Disconnect all cables from the lock-in. Turn the
power on while holding down the [Local] key.
The power switch is on the power entry module
on the rear panel.

2. Connect the Sine Out + on the front panel to the
A input using a BNC cable.

3. Touch {Ampl} in the info bar along the top of
the screen. Then {5}{0}{0}{mV}.

When the power is turned on with [Local]

pressed, the lock-in returns to its standard

the Operation section for a complete listing of

The lock-in defaults to the internal oscillator

reference set at 100.000 kHz. The reference
source is indicated by the
mode, the lock-in generates a synchronous sine
output at the internal reference frequency.

The default data screen is the Trend Graph. The

4 displayed parameters default to X, Y, R and θ.

Each parameter has a numeric display, a bar
graph and a trend graph. The trend graph is a
continuously autoscaling graph of the recent
history of each parameter. This data screen has
no adjustments available.

The default sine amplitude is 0 Vrms. Thus the

data displays will read 0 for X, Y and R. θ will

be just noise.

The lock-in parameters shown in the info bar at

the top of the screen may be entered using a
numeric keypad simply by touching them.

Internal LED. In this

4. Press the [Auto Phase] key. Automatically adjust the reference phase shift

SR865 DSP Lock-in Amplifier

The Sine Out amplitude is specified for
differential output (Sine+) − (Sine−). In this
case, each BNC has an amplitude of 250 mV
(rms) with a 50Ω output. The lock-in input is
high impedance so the output of each BNC is
doubled and the lock-in measures 500 mV.

Since the phase shift of the sine output is very
close to zero, X (green) should read about

0.5000 and Y (blue) should read close to

0.0000 V.

to eliminate any residual phase error. This

10 Getting Started Chapter 1

the magnitude (−0.5000 V).

touchscreen keypad.

adjusted as well.

Range to maximize the signal at the A/D

5. Press the [+90º] key. This adds 90º to the reference phase shift. The
value of X drops to zero and Y becomes minus

Use the <Phase> knob to adjust the phase shift

back to zero (press and hold the <Phase> knob
inward as a short cut). The Phase shift is
displayed in the info bar at the top of the screen.

6. Touch {Fint} in the info bar.

The lock-in parameters shown in the info bar,

Phase, Reference Frequency, Detected
Harmonic, Sine Amplitude and Offset, can all
be adjusted from the front panel as well as via a

Display the numeric entry screen for internal

reference frequency. The 4 buttons labelled
{F1} thru {F4} are frequency presets. Press and
hold them to memorize new frequencies.

Touch {1}{0}{kHz} to enter a new frequency. Change the frequency to 10 kHz.

Use the <Frequency> knob to adjust the

frequency to 1.00000 kHz.

The knob is very useful for making small

adjustments or optimizing a setting. Large
changes are better left to the numeric keypad.

The measured signal amplitude X and R should
stay within 1% of 500 mV and Y and θ should
stay close to zero.

7. Use the <Amplitude> knob to adjust the sine out

to 5.0 mV. The Amplitude is displayed in the
info bar.

As the amplitude is changed, the values of X

and R change to follow.
The yellow LED in the Input Range section

should light. The Input Range is the largest
input signal before overload. The lower the
range, the higher the gain. The signal strength
indicates how much of the A/D converter range
is being used. When the yellow indicator lights,
it means that more gain should be used.

Since the signal has just been reduced by a
factor of 100, the input range should be

8. Press [Auto Range]. The Auto Range function changes the Input

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 11

converter without overload. In this case the

Input Range should change to 100 mV.

The Sensitivity is indicated with 3 LEDs. In this

resolution for the smaller signal.

Input Range should change to 10 mV.
The Input Range is the peak allowable voltage

at the input, whether noise or signal. In this
case, the signal is 5 mVrms or 7 mVpk so
10 mV is the best allowed setting.

The signal strength increases from the
minimum (yellow) to something in the middle.

9. Use the <Amplitude> knob to increase the sine
out to 50.0 mV. The Amplitude is displayed in
the info bar.

The peak signal exceeds the input range so the

Input Range

Overload LED lights. Ovld

indicators also appear on the screen when a
displayed value is invalidated by an input
overload.

10. Press [Auto Range]. During Input Range Overload, the Auto Range

function selects the 1 V range.

Press [Auto Range] again. From the 1 V range, the Auto Range function

changes the Input Range to maximize the signal
at the A/D converter without overload. The

10. Press [Input Range Down] to select 30 mV. Settings which have many options, such as

Input Range, Time Constant and Sensitivity, are
changed with up and down keys. The setting is
indicated by LEDs.

The peak signal exceeds the input range so the
Input Range

Overload LED lights. Ovld

indicators also appear on the screen when a
displayed value is invalidated by an input
overload.

Press [Input Range Up] to select 100 mV.

11. Press [Sensitivity Down] multiple times to select
50 mV.

12. Press [Time Constant Down] multiple times to The Time Constant is indicated with 3 LEDs. In

SR865 DSP Lock-in Amplifier

case, the 5, ×10 and mV should be lit.
The Sensitivity sets full scale for the bar graphs

and the resolution for the numeric readouts for
X, Y and R. Sensitivity is also the signal
reading corresponding to 10 V on the CH1 and
CH2 outputs (for X, Y and R).

By decreasing the scale value, the bar graphs
and numeric readings display much more

12 Getting Started Chapter 1

select 300 μs.

this case, the 3, ×100 and μs.

value.

reasonably well and provides steady readings.

noisy for now.

The output values become noisy. This is
because the 2f component of the output (at
2 kHz) is no longer attenuated completely by
the low pass filter.

The red Output Overload LED for CH1 will
light indicating that the output voltage is
clipping. The 50 mV signal outputs 10 V when
the sensitivity is 50 mV. The large additional 2f
component will cause the output to try and
exceed 10 V and results in an output overload.
Output overload does not affect the actual
displayed value, it just indicates that the CH1
(or CH2) output is not following the measured

12. Press [Slope] to select 12 dB/oct. Parameters which have only a few values, such
as Filter Slope and External Source, have only a
single key which cycles through all available
options. Press the key until the desired option is
indicated by an LED.

Press [Slope] twice more to select 24 dB/oct. With 4 poles of low pass filtering, even this

Press [Slope] again to select 6 dB/oct. Let's leave the filtering short and the outputs

13. Press [Sync] to turn on synchronous filtering. This turns on synchronous filtering whenever

The outputs are less noisy with 2 poles of
filtering.

short time constant attenuates the 2f component

the detection frequency is below 4.8 kHz.
Synchronous filtering effectively removes

output components at multiples of the detection
frequency. At low frequencies, this filter is a
very effective way to remove 2f without using
extremely long time constants.

The outputs are now quiet and steady, even
though the time constant is very short. The
response time of the synchronous filte r is equal
to the period of the detection frequency (1 ms in
this case).

This concludes this measurement example. You

SR865 DSP Lock-in Amplifier

should have a feeling for the basic operation of
the front panel. Basic lock-in parameters have
been introduced and you should be able to
perform simple measurements.

Chapter 1 Getting Started 13

Do This

Explanation

ttings. See the Sta n da rd S et tin g s list in

the settings.

setting.

phase independent and does not change.

Using Displays

This measurement is designed to use the internal oscillator and an external signal source to explore some
of the display types. You will need a synthesized function generator capable of providing a 500 mVrms
sine wave at 100.000 kHz, BNC cables and a terminator appropriate for the generator function output.

Specifically, you will display the lock-in outputs when measuring a signal close to, but not equal to, the
internal reference frequency. This setup ensures changing outputs which are more illustrative than steady
outputs.

1. Disconnect all cables from the lock-in. Turn the
power on while holding down the [Local] key.
The power switch is on the power entry module
on the rear panel.

2. Turn on the function generator, set the frequency
to 100.000 kHz (exactly) and the amplitude to
500 mVrms.

Connect the function output (sine wave) from
the synthesized function generator to the A input
using a BNC cable and appropriate terminator.

When the power is turned on with [Local]

pressed, the lock-in returns to its standard
default se
the Operation section for a complete listing of

The input impedance of the lock-in is 10 MΩ.

The generator may require a terminator. Many

generators have either a 50Ω or 600Ω output

impedance. Use the app ropriate feedthroug h or
T termination if necessary. In general, not using
a terminator means that the function output
amplitude will not agree with the generator

The default screen is the Trend Graph. Four

data channels are displayed as values, bar and
trend graphs. The trend graph is the recent
history of each data channel with continuous
auto-scaling. In this case R (yellow) auto scales
to show the tiny amount of noise in the signal
magnitude. Trend graphs have no adjustments
and are most useful when adjusting an
experiment to find a maximum or minimum.

SR865 DSP Lock-in Amplifier

The lock-in defaults to the internal oscillator
reference set at 100.000 kHz. The reference
source is indicated by the

The internal oscillator should be very close to
the actual generator frequency . The X (green)
and Y (blue) displays should read values which
change slowly. The lock-in and the generator
are not phase locked but they are at (nearly) the
same frequency with a slowly changing θ
(orange). The signal magnitude R (yellow) is

Internal LED.

14 Getting Started Chapter 1

per division for the 4 data channels (green, blue,

scale per division (white).

scale of the selected data channel’s trace. Touch

3. Use the <Frequency> knob to carefully adjust
the frequency to 99.9998 kHz. That’s 0.2 Hz
below 100 kHz.

4. Press [Screen Layout] once to change the display
to the full screen strip chart.

By setting the lock-in reference 0.2 Hz away

from the signal frequency, the X and Y outputs
are 0.2 Hz sine waves (difference between f
and f

). The X and Y displays should now

sig

ref

oscillate at about 0.2 Hz (the accuracy is
determined by the timebases of the generator
and the lock-in).

The most common way to visualize the lock-in

outputs is to use the strip chart display. New
data is plotted at the right edge and older data
scrolls left. The scroll rate is determined by the
horizontal scale (time per division). Th e fastest
rate is 0.5 s/div and the shows 5 s of history.

5. Touch the orange scale tile {θ} at the bottom to
display the vertical scale palette.

The info bar is at the top of the screen. Touch a
tile to change a parameter using a keypad.

The numeric and bar graph displays shrink to fit
above the chart.

The scale bar is shown below the strip chart.
This bar shows tiles ind icat ing the vertical sca le

yellow and orange) and the horizontal time

Use the palette functions to adjust the ver tical

the scale tile again to dismiss the palette.
The trace may be moved up and down, auto

scaled, zoomed in and out and dismissed
entirely.

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 15

its palette leaving all palettes off.

Touch to auto scale the blue Y trace.

Touch to auto scale the orange θ graph.

6. Touch the green scale tile {X} to display its
scale palette.

Touch to auto scale the green X trace.

7. Touch the blue scale tile {Y} to display its scale
palette.

Touch to auto scale a trace. Touch to
auto scale while keeping zero in the center.

The phase is ramping from −180º to +180º so
the resulting scale is 50/div for a graph of
±200º.

Touch the orange scale tile again to dismiss the
scale palette.

Selecting a scale tile automatically dismisses

any other palette.
Touch a highlighted scale tile to simply dismiss

The X and Y outputs are 0.2 Hz sine waves

with 500 mV amplitudes.

8. Touch the yellow scale tile {R} to display its
scale palette.

Touch to move the trace so the newest
points are vertically centered.

Touch repeatedly to zoom in about the
center. Keep zooming in until the yellow trace
shows some noise. The scale will probably end
up less than 1 mV.

The magnitude R is phase independent and is a

straight flat line at about 500 mV.
By centering the trace, the zoom function will

expand the trace to reveal noise on R.
The little triangles along the right edge indicate

the zero for each data channel. Zooming in on R
moves the zero for R below the graph as
indicated by the downward facing yellow
triangle at the bottom right edge.

SR865 DSP Lock-in Amplifier

16 Getting Started Chapter 1

up and down.

Touching within the graph area when no trace is

Zooming changes the horizontal scale and scroll

‘Ovld’ indicators are displayed for X, Y, R and

9. Touch and drag in the graph area while the
yellow scale palette is displayed to move the
yellow trace up and down.

10. Touch the highlighted yellow scale tile {R} to
dismiss its scale palette.

Now touch anywhere within the graph area.

When a trace is selected (by displaying its scale

palette) touching anywhere inside the graph
area (and not a scale button) drags the trace up
and down.

The and buttons also move the trace

selected turns on a status display across the top
of the graph. This displays the lock-in signal
settings.

This status is useful when the HDMI port (on
the rear panel) is used to drive an external
monitor or TV. Users who are looking at the
monitor can see the lock-in front panel settings.

Touch anywhere within the graph area to dismiss

the status display.

11. Touch the white scale tile {Time} to display the
horizontal scale palette.

Touch repeatedly to zoom out.

12. Increase the amplitude of the function generator
to 1.5 Vrms.

The status display is dismissed when the graph

area is touched or a scale palette is displayed.
Simply turn it back on with a touch if desired.

Horizontal scale changes are applied to the

entire strip chart display and all data channels.

speed. The chart always displays the most
recent point at the right edge.

In this case, zooming out displays more history
and more cycles of X, Y and θ appear.

The signal now exceeds the input range of 1 V

(peak) so the Input Range

Overload LED is on.

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 17

θ because these values are affected by signal

Conditions such as overload or reference unlock

that the numeric and bar graphs are full sized.

overload.

are displayed in violet along the bottom. This
provides visual feedback about the validity of
the data in those regions.

Decrease the amplitude of the function generator

back to 500 mVrms.

13. Touch the white scale tile {Time} to display the
horizontal scale palette.

Touch repeatedly to zoom out.

Touch repeatedly to zoom back in.

14. Press [Screen Layout] once to change the display
to the half screen strip chart.

The overload condition goes away.

Zoom out on the horizontal time scale to show

more and more history. When the region where
the signal was overloaded is shown, the
overload is ind ica ted by the violet points alo n g
the bottom edge.

The half screen strip chart behaves the same as

the full screen version. The only difference is

SR865 DSP Lock-in Amplifier

18 Getting Started Chapter 1

at the bottom of the graph. The cursor readout is

as the full screen version. The only difference is
that the numeric and bar graphs are full sized.

15. Press [Screen Layout] again to change the
display to the full screen FFT.

16. Press [Screen Layout] again to change the
display to the half screen FFT.

17. Press [Screen Layout] again to change the
display to the full numeric display.

The FFT of the signal input is displayed. There

is only a single quantity shown.
The left and right edge frequencies are label led

at the right.
The display is adjusted with the tiles across the

bottom. {Src} selects the source data for the
FFT. {dB} and {Hz} adjust the vertical and
horizontal scales. {Avgs} sets the amount of
averages and {Live} toggles to {Paused}.

Use the <Cursor> knob to move the cursor.

The half screen FFT display behaves the same

The full numeric screen adds readouts and bar

graphs for the 4 aux inputs on the rear panel.
The 4 aux outputs are shown in tiles across the
bottom. Touching an output tile displays a
keypad to set the aux output.

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 19

the display screens.

18. Press [Screen Layout] again to cycle back to the
trend graph.

Use [Screen Layout] to cycle through the

various display screens.

This concludes the measurement example. You

should have a feeling for the basic operation of

SR865 DSP Lock-in Amplifier

20 Getting Started Chapter 1

Do This

Explanation

default settings. See the Standard Settings list in

be just noise.

Sensitivity, Offset and Expand

This measurement is designed to use the internal oscillator to explore some of the basic lock-in outputs.
You will need BNC cables and a digital voltmeter (DVM).

Specifically, you will measure the amplitude of the Sine Out and provide analog outputs proportional to
the measurement. The effect of offsets and expands on the displayed values and the analog outputs will be
explored.

1. Disconnect all cables from the lock-in. Turn the
power on while holding down the [Local] key.
The power switch is on the power entry module
on the rear panel.

2. Connect the Sine Out + on the front panel to the
A input using a BNC cable.

3. Touch {Ampl} in the info bar along the top of
the screen. Then {1}{0}{0}{mV}.

When the power is turned on with [Local]

pressed, the lock-in returns to its standard

the Operation section for a complete listing of
the settings.

The lock-in defaults to the internal oscillator

reference set at 100.000 kHz.
The default data screen is the Trend Graph. The

4 displayed parameters default to X, Y, R and θ.

Each parameter has a numeric display, a bar
graph and a trend graph. The trend graph is a
continuously autoscaling graph of the recent
history of each parameter. This data screen has
no adjustments available.

The default sine amplitude is 0 Vrms. Thus the
data displays will read 0 for X, Y and R. θ will

The lock-in parameters shown in the info bar at

the top of the screen may be entered using a
numeric keypad simply by touching them.

SR865 DSP Lock-in Amplifier

The Sine Out amplitude is specified for
differential output (Sine+) − (Sine−). In this
case, each BNC has an amplitude of 50 mV
(rms) with a 50Ω output. The lock-in input is
high impedance so the output of each BNC is
doubled and the lock-in measures 100 mV.

Since the phase shift of the sine output is very
close to zero, X (green) should read about

0.1000 and Y (blue) should read close to

0.0000 V.

Chapter 1 Getting Started 21

10VExpandOffset

ySensitivit

X

CH1 ××















−=

for the bar graphs of X, Y and R.

screen strip chart.

different displays.

4. Connect the CH1 Output on the front panel to
the DVM. Set the DVM to read dc Volts.

5. Press [Screen Layout] twice to show the half

6. Touch the green scale tile {X} to display its
scale palette.

Touch to auto scale the green X trace.

The CH1 output defaults to X. The output

voltage (with ratio disabled) is given by:

In this case, X = 0.1 V, Sensitivity = 1 V, the
offset is zero percent and the expand is 1. The
output should thus be 1 V or 10% of full scale.

Note that the bar graph for X (and R) is at
+10%. The Sensitivity (1 V) sets the full scale

Now let’s look at how the Sensitivity affects the

auto scales the trace keeping zero at the
center. The zero location is indicated by the
small green triangle on the right edge. The scale
is 50 mV/div so the green data is a line 2
divisions above the center.

Touch the green highlighted scale tile again to
dismiss the scale palette.

7. Press [Sensitivity Down] to select 500 mV.

SR865 DSP Lock-in Amplifier

The DVM should now read 2 V. This is because

X (100 mV) is now 20% of the sensitivity
(500 mV). This also increases the bar graph to
+20% and increases the resolution of the
numeric readout.

22 Getting Started Chapter 1

should be about 100%. Offsets are set as a

8. Press [Sensitivity Down] two more times to
select 100 mV.

Note that the trace of X is unchanged. This is

because the value of X is unchanged.
The Sensitivity does not af fect the valu e of the

output, just the way the value is scaled to the
displays and analog output.

The Sensitivity applies to X, Y and R.

The DVM should now read 10 V and X is now

100% of full scale on the bar graph.
It is important to adjust the Sensitivity even if

the analog outputs are not being used. The
Sensitivity determines the resolution of the
numeric readouts and bar graphs.

[Auto Scale] will adjust the Sensitivity
automatically.

9. Press and hold the <CH1 Offset> knob (above
the CH1 BNC) to display the offset keypad.

X, Y and R may all be offset, ratioed, and

expanded separately.
Since CH1 is set to X (indicated by the

X LED

above the [Select] key) the <CH1 Offset> knob
and [Expand] key above the CH1 BNC set the
X offset and expand.

The [CH1 Select] key determines which
quantity (X or R) is offset, ratioed, or expanded,
and output on the BNC.

The ratio function is described later in this
manual, in the Operation chapter. Here we will
explore offset and expand.

10. Touch {Auto} in the offset keypad screen. Auto Offset automatically adjusts the X offset

(or Y or R) such that X (or Y or R) becomes
zero. In this case, X is offset to zero. The offset

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 23

percentage of the Sensitivity up to 999% (10×).

graph and numeric value are both zero.

sensitivity).

Press it again to turn it back on.

Leave the X offset on for now.

10VExpandOffset

ySensitivit

X

CH1 ××















−=

expand does not change the value of X.

Offsets are useful for making relative
measurements.

Offsets can also be set using the keypad or
<Offset> knob.

The offset affects the value of X and any
outputs or displays of X. The DVM voltage
should be zero in this case.

The Offset indicator turns on next to the <CH1
Offset> knob. The X display on the screen has
an ‘Ofst’ indication that the displayed quantity
is affected by an offset. In this case, the bar

11. Touch {9}{0}{Enter} to set the offset to 90%. The X output (10 mV) is now 10% of the
sensitivity (100 mV). The bar graph is at 10%
and the DVM reads 1 V.

Notice that the trace of X (green) is a line
10 mV above zero (center). This is because the
offset affects the value of X (unlike the

12. Press the <CH1 Offset> knob briefly once to

turn the X offset off.

13. Press [CH1 Expand] once to select ×10. Expand ×10 e ffect iv ely decreas es the sensitivity

The offset for CH1 can be turned on and off

without changing the offset value. Notice how
the trace of X changes when the offset is turned
off.

by 10 after the offset is applied.

Now, X = 100 mV, Sensitivity = 100 mV, the
offset is 90% and the expand is ×10. Thus the
DVM reads 10 V.

The X bar graph is now at 100% and the
numeric readout has added resolution. The X
display has an ‘Expd’ indication that the
displayed quantity is affected by a non-unity
expand.

Expand increases the resolution of the X
display and CH1 output. Note that the trace of
X is unaffected by expand. This is because

SR865 DSP Lock-in Amplifier

24 Getting Started Chapter 1

overloaded. This has no affect on the value of X

turn the X offset off.

values of X, Y and R.

output scaling

14. Press [CH1 Expand] once to select ×100. The red output Overload LED lights and a
‘Scale’ overload is indicated in the X display.
This is because CH1 is trying to reach 100 V
(10 times the previous DVM reading). Since the
CH1 output is limited to 10 V, the output is

and the trace is unchanged. The bar graph and
displayed value are pinned howev er.

15. Press [CH1 Expand] once to turn off expand.

Press the <CH1 Offset> knob briefly once to

The X display returns to 100 mV, 100% bar

graph and 10 V CH1 output. The X graph is a
line at 100 mV 2 divisions above center.

With offset and expand, the output voltage gain

and offset can be programmed to provide
control of feedback signals with the prope r bias
and gain for a variety of situations.

Offsets add and subtract from the values of X,
Y and R.

Expand increases the resolution of the displays
and analog outputs but does not change the

16. Touch the green scale tile {X} to display its

scale palette.

When using the strip chart graph exclusively,

there is no need to use offset or expand to zoom
in on the data. Simply auto scale the data

Touch to auto scale the green X trace.

channel to graphically offset and expand the
chart data.

See the Outputs and Scales discussion in the

SR865 DSP Lock-in Amplifier

next chapter for more detailed information on

Chapter 1 Getting Started 25

Do This

Explanation

ttings. See the Sta n da rd S et tin g s list in

the settings.

Filter Slope.

Saving and Recalling Setups

The SR865 can store 8 complete instrument setups in non-volatile memory.

1. Disconnect all cables from the lock-in. Turn the
power on while holding down the [Local] key.
The power switch is on the power entry module
on the rear panel.

2. Press and hold [Calc/system] to display the
system menu.

3. Touch the {h} button in the Time section to
highlight the hours setting. Use the keypad to set
the hour of day in 24 hour format. The time/date
will highlight in orange indicating that the
displayed time is not the current time but rather
the time to be set.

When the power is turned on with [Local]

pressed, the lock-in returns to its standard
default se
the Operation section for a complete listing of

First let’s set the SR865 clock.

The system menu is where instrument
parameters (not measu rement parameters) are
set. This includes file numbering, interface
settings and software updates.

The time and date are used to label data files,

screen shots, and saved settings.

Continue to enter the minutes, seconds then
touch {Time set} to commit the time to the
internal clock.

Set the date in the same manner touching {Date
set] to commit the date.

4. Press [Sensitivity Down] 3 times to select
100 mV.

Press [Time Constant Up] twice to select 1 s.

SR865 DSP Lock-in Amplifier

Let’s change the lock-in setup so that we have a

non-default setup to save.
Change the Sensitivity, Time Constant and

26 Getting Started Chapter 1

Press [Filter Slope] once to select 12 dB/oct.

Setups are numbered 1 through 8. Setups should

Filter Slope to new settings.

saved settings.

5. Press [Save Recall] to display the Save/Recall
screen.

6. Touch the {Save} button next to the large tile
labelled ‘1’.

The SR865 can store 8 complete setups. In

addition the default setup can be recalled.

be named so they are easily distinguished.

Enter a name for this setup using the keypad.
Touch {Confirm} to commit the current setup to

location 1.

7. Now change the Sensitivity, Time Constant and

8. Press [Save Recall] to display the Save/Recall
screen again.

SR865 DSP Lock-in Amplifier

Change the lock-in setup before recalling the

Note that the Location 1 tile displays the setup

name and the time and date it was created. This
makes it easier to recall the correct setup.

Chapter 1 Getting Started 27

this screen.

should all return to the saved settings.

9. Touch {Recall} for Location 1.

Touch {Confirm} to recall the setup and dismiss

A summary of settings which will change upon

recall is shown.
Simply touch {Cancel} to skip recalling this

setup.

The Sensitivity, Time Constant and Filter Slope

SR865 DSP Lock-in Amplifier

28 Getting Started Chapter 1

Do This

Explanation

default settings. See the Standard Settings list in

the settings.

the computer interface.

4.

Touch {1}{0}{Enter} to set Aux 1 to 10.000 V.

The DVM should display 10.00 V.

Aux Outputs and Inputs

This measurement is designed to illustrate the use of the Aux Outputs and Inputs on the rear panel. You
will need BNC cables and a digital voltmeter (DVM).

Specifically, you will set the Aux Output voltages and measure them with the DVM. These outputs will
then be connected to the Aux Inputs to simulate external dc voltages which the loc k-in can measure.

1. Disconnect all cables from the lock-in. Turn the
power on while holding down the [Local] key.
The power switch is on the power entry module
on the rear panel.

2. Connect Aux Out 1 on the rear panel to the
DVM. Set the DVM to read dc volts.

3. Press [Aux Output] to show the Aux Output
keypad.

When the power is turned on with [Local]

pressed, the lock-in returns to its standard

the Operation section for a complete listing of

The 4 Aux Outputs can provide programmable

voltages between −10.5 an d +10.5 volts. The
outputs can be set from the front panel or via

Aux Outputs are easily set from the front panel.

Use the <Cursor> knob adjust the level to

5.000 V.

SR865 DSP Lock-in Amplifier

The <Cursor> knob is used to adjust values

when the Aux Output keypad is shown.
The DVM should display 5.00 V.
The 4 Aux Outputs are useful for controlling

other parameters in an experiment, such as
pressure, temperature, wavelength, etc.

Chapter 1 Getting Started 29

output tile will display the Aux Output keypad.

to change a data channel.

5. Touch [Screen Layout] multiple times to show
the full numeric display screen.

Note the 4 Aux Output values are displayed in
white tiles across the bottom. Touching an

6. Disconnect the DVM from Aux Out 1. Connect
Aux Out 1 to Aux In 1 on the rear panel.

This screen displays the 4 Aux Input readings

along with the 4 lock-in data channels. The Aux
Inputs are always scaled to 10 V.

The Aux Inputs can read 4 analog voltages.
These inputs are useful for monitoring and
measuring other parameters in an experiment,
such as pressure, temperature, position, etc.

We'll use Aux Out 1 to provide an analog

voltage to measure.
Aux In 1 should now read 5.000 V.

The Aux Inputs can be assigned to a data

channel and graphed on the strip chart
alongside lock-in outputs. Use the [Config] key

SR865 DSP Lock-in Amplifier

30 Getting Started Chapter 1

Do This

Explanation

default settings. See the Standard Settings list in

the settings.

phase independent and does not change.

the fewer steps. With experiments that take time

Scanning

This measurement is designed to use the internal oscillator and an external signal source to explore some
of the display types. You will need a synthesized function generator capable of providing a 500 mVrms
sine wave at 100.000 kHz, BNC cables and a terminator appropriate for the generator function output.

Specifically, you will scan the lock-in internal reference frequency through the signal frequency.

1. Disconnect all cables from the lock-in. Turn the
power on while holding down the [Local] key.
The power switch is on the power entry module
on the rear panel.

2. Turn on the function generator, set the frequency
to 100.000 kHz (exactly) and the amplitude to
500 mVrms.

Connect the function output (sine wave) from
the synthesized function generator to the A input
using a BNC cable and appropriate terminator.

When the power is turned on with [Local]

pressed, the lock-in returns to its standard

the Operation section for a complete listing of

The input impedance of the lock-in is 10 MΩ.

The generator may require a terminator. Many

generators have either a 50Ω or 600Ω output

impedance. Use the appropriate feedthrough or
T termination if necessary. In general, not using
a terminator means that the function output
amplitude will not agree with the generator
setting.

The internal oscillator should be very close to
the actual generator frequency. The X (green)
and Y (blue) displays should read values which
change slowly. The lock-in and the generator
are not phase locked but they are at the same

frequency with some slowly changing θ

(orange). The signal magnitude R (yellow) is

3. Press and hold [Scan/setup] to display the scan
menu.

SR865 DSP Lock-in Amplifier

Parameters which may be scanned are F

Amplitude, DC Level, Aux Out 1 and 2.
Scans can be linear or logarithmic, repeat,

repeat up and down or run once and pause.
The Scan Duration is the total time to move

from the Begin Value to the End Value. The
Parameter Update Interval is the time spent at
each scan step along the way. The shorter the
update time, the smaller the steps and the
smoother the scan. The longer the update time,

to settle after a parameter change, it can be
beneficial to set the update time long enough to
accommodate the settling.

internal

,

Chapter 1 Getting Started 31

the end of each scan.

not been started.

start value or 99.9900 kHz.

frequency when it is large.

Touch repeatedly to zoom out to 10 S/div.

4. Let’s leave the Scan Parameter at internal
reference frequency.

Touch {Begin Value} to highlight the start
frequency. Enter 99.990 kHz.

Touch {End Value} to highlight the stop
frequency. Enter 100.010 kHz.

Let’s scan a 20 Hz span around the signal

frequency (100.000 kHz).
This setup will increase the internal frequency

from 99.990 kHz to 100.010 kHz. To reverse
the scan, simply reverse the begin and end
values.

5. Touch {End Mode} once to select Repeat. This will repeat the scan over and over jumping

from the end value back to the begin value at

6. Touch {Close} to return to the Trend Graphs. The lock-in frequency has not been changed,

we have only configured a scan. The scan has

7. Press [Scan/setup] briefly (don’t hold it) to turn
on the scan.

The Ready LED turns on indicating that the

scan is ready (at the start value). In this case,
the internal frequency (in the info bar at the top)
is shown in orange indicating that it is under
scan control. At this time, the frequency is the

8. Press [Play Pause/reset] briefly (don’t hold it) to
start the scan.

9. Press [Screen Layout] once to show the full
screen strip chart.

Touch the white scale tile {Time} to display the
horizontal scale palette (bottom right of screen).

The Run LED turns on indicating that the scan

is running.
The internal frequency starts scanning up. The

current value is shown in orange in the info bar.
When the end value is reached, the scan resets
to the beginning value and repeats.

The trend graphs show a resonance as the
internal frequency passes through 100.000 kHz.
This is because the difference between the f
and f
f

signal

gets slower as the f

internal

approaches

internal

and then gets faster after it passes f

signal

signal

.

The time constant attenuates this difference

It is much easier to visualize a scan using the

strip charts since the chart time scale can be
matched to the scan time.

This scan takes 1m40s or 100 s so a chart span
of 100 s will display an entire scan.

SR865 DSP Lock-in Amplifier

32 Getting Started Chapter 1

palette. Touch to dismiss the Y trace.

chart by dismissing the X and

the scale to 200 deg/div.

10. Touch the green scale tile {X} to display its
scale palette. Touch to dismiss the X trace.

Touch the blue scale tile {Y} to display its scale

11. Touch the yellow scale tile {R} to display its
scale palette. Touch to auto scale the R

trace.

Touch the orange scale tile {θ} to display its

scale palette. Touch to repeatedly to change

12. Press [Config] to change the assignm ents of the
data channels.

Touch {Data 2 Display} (the blue display) to
highlight the channel 2 data source. Touch
{Fint} from the keypad below.

Let’s clean up the

Y traces.

The magnitude trace (R) shows the lock-in

response as the internal frequency scans
through the signal frequency at this time
constant and filter.

Reduce the phase trace to see the resonance at

100.000 kHz.

It would be nice to show the frequency on the

graph.
The 4 data channels can be assigned to differen t

parameters in the Config screen.
Any data channel can be assigned any of the

sources in the keypad.
When the Config screen is closed the strip chart

does not display a blue trace. This is because
we dismissed it previously.

Touch {Close} to return to the strip chart.

SR865 DSP Lock-in Amplifier

Chapter 1 Getting Started 33

button; the Ready LED goes off and then on.

13. Touch the blue scale tile {Fi} to display its scale
palette.

Touch to auto scale the blue F trace.

14. Touch the white scale tile {Time} to display the
horizontal scale palette (bottom right of screen).

Touch repeatedly to zoom out a few times.

Simply touching the blue scale tile turns the

blue trace back on and displays its scale palette.
The blue frequency trace shows the upward

scan of the frequency spanning 4 divisions or
20 Hz.

Since the scan End Mode is set to repeat, we

see the scan repeat over and over in the history.
When scanning, it is convenient to pause the

strip chart at the end of the scan to review the
results.

15. Press and hold [Scan/setup] to display the scan
menu again.

Touch {End Mode} multiple times to select
Once.

Touch {Close} to return to the strip chart.

16. Press [Scan/setup] briefly (don’t hold it) to turn
the scan off, then press [Scan/setup] briefly
again to re-arm.

SR865 DSP Lock-in Amplifier

When the End Mode is Once, then the scan

stops at the end value and the strip chart is
paused.

Changes to the scan setup do not affect a

currently-running scan. We stop the scan and
then start a new one by cycling the [Scan/setup]

34 Getting Started Chapter 1

dismiss the scale palette.

on.

examining the paused scan.

scan back to the begin value.

value. The Done LED turns off.

17. Press [Play Pause/reset] briefly (don’t hold it) to
re-start the scan.

18. Touch the white scale tile {Time} to display the
horizontal scale palette (bottom right of screen).

Touch repeatedly to zoom in to 10 S/div.

The Run LED turns on indicating that the scan

is running.

This scan is 100 s in length so set the graph to

10 divisions of 10 s to show a complete scan.

Touch the highlighted white scale tile again to

Wait for the scan to finish. When the scan finishes, the Done LED turns

19. Use the <Cursor> knob to move the cursor to the
peak of R (yellow).

When the scan in progress reaches the end

value the strip chart pauses.
The frequency will hold at the end value

(100.010 kHz) as displayed in the info bar.
The cursor is active when the strip chart is

paused.

20. Touch the white scale tile {Time} to display the
horizontal scale palette (bottom right of screen).

Touch start the strip chart again.
Touch the highlighted white scale tile again to

dismiss the scale palette.

21. Press and hold [Play Pause/reset] to reset the

22. Press [Play Pause/reset] briefly to start the scan
again.

SR865 DSP Lock-in Amplifier

Notice that data collection continued while the

chart was paused. Restarting the chart realigns
the time history so the cu r r en t time is the right
edge again.

In this case, the frequency has been constant at

100.010 kHz the entire time we have been

This resets the scan parameter back to the begin

This starts the scan again. The Run LED turns

on and the frequency ramps upward.

Chapter 1 Getting Started 35

While the scan is in progress, press [Scan /setup]

briefly to turn scanning off.

Turn the scan off before the end and the internal

frequency returns to its original value
(100.000 kHz) as shown in white in the info
bar.

SR865 DSP Lock-in Amplifier

36 Getting Started Chapter 1

SR865 DSP Lock-in Amplifier

Basics 37

Chapter 2

Lock-in Amplifier Basics

What is a Lock-in Amplifier?

Lock-in amplifiers are used to detect and measure very small ac signals — all the way
down to a few nanovolts. Accurate measurements may be made even when the small
signal is obscured by noise sources many thousands of times larger.

Lock-in amplifiers use a technique known as phase-sensitive detection to single out the
component of the signal at a specific reference frequency and phase. Noise signals at
frequencies other than the reference frequency are rejected and do not affect the
measurement.

Why use a lock-in?

Let's consider an example. Suppose the signal is a 10 nV sine wave at 10 kHz. Clearly
some amplification is required. A good low noise amplifier may have about 5 nV/√Hz of
input noise. If the amplifier bandwidth is 100 kHz and the gain is 1000, then we can
expect our output to be 10 μV of signal (10 nV × 1000) and 1.6 mV of broadband noise
(5 nV/√Hz × √100 kHz × 1000). We won't have much luck measuring the output signal
unless we single out the frequency of interest.

If we follow the amplifier with a band pass filter with a Q=100 (a very good filter)
centered at 10 kHz, any signal in a 100 Hz bandwidth will be detected (10 kHz/Q). The
noise in the filter pass band will be 50 μV (5 nV/√Hz × √100 Hz × 1000) and the signal
will still be 10 μV. The output noise is still much greater than the signal and an accurate
measurement cannot be made. Further gain will not help the signal to noise problem.

Now try following the amplifier with a phase-sensitive detector (PSD). The PSD can
detect the signal at 10 kHz with a bandwidth as narrow as 0.01 Hz! In this case, the noise
in the detection bandwidth will be only 0.5 μV (5 nV/√Hz × √.01 Hz × 1000) while the
signal is still 10 µV. The signal to noise ratio is now 20 and an accurate measurement of
the signal is possible.

What is phase-sensitive detection?

Lock-in measurements require a frequency reference. Typically an experiment is excited
at a fixed frequency (from an oscillator or function generator) and the lock-in detects the
response from the experiment at the reference frequency. In the diagram below, the
reference signal is a square wave at frequency f
function generator. If the sine output from the function generator is used to excite the
experiment, the response might be the signal waveform shown below. The signal is
V

sin(ω

sig

ref

t + θ

) where ω

sig

ref

= 2πf

and V

ref

. This might be the sync output from a

ref

is the signal amplitude.

sig

SR865 DSP Lock-in Amplifier

38 Basics Chapter 2

Lock-in Reference

External Reference

θ

sig

θ

ref

Signal

The SR865 generates its own sine wave at frequency fL, shown as the lock-in reference
below. The lock-in reference is sin(ω

The SR865 amplifies the signal and then multiplies it by the lock-in reference using a
phase-sensitive detector (PSD) or multiplier. The output of the PSD is simply the product
of two sine waves.

t + θ

L

) where ωL = 2πfL.

ref

V

= 1/2 V

1/2 V

The PSD output is two ac signals, one at the difference frequency (ω

at the sum frequency (ω

= V

psd

sin(ωrt + θ

sig

cos([ωr − ωL]t + θ

sig

cos([ωr + ωL]t + θ

sig

+ ωL).

r

) sin(ωLt + θ

sig

sig

sig

ref

− θ

+ θ

)

) −

ref

)

ref

− ωL) and the other

r

If the PSD output is passed through a low pass filter, the ac signals are removed. What

will be left? In the general case, nothing. However, if ω

equals ωL, the difference

r

frequency component will be a dc signal. In this case, the filtered PSD output will be

V

= 1/2 V

psd

cos(θ

sig

sig

− θ

)

ref

This is a very nice signal — it is a dc signal proportional to the signal amplitude.

Narrow band detection

Now suppose the input is made up of signal plus noise. The PSD and low pass filter only
detect signals whose frequencies are very close to the lock-in reference frequency. Noise
signals at frequencies far from the reference are attenuated at the PSD output by the low

pass filter (neither ω

close to the reference frequency will result in very low frequency ac outputs from the

PSD (|ω

noise

− ω

ref

and roll-off. A narrower bandwidth will remove noise sources very close to the reference
frequency, a wider bandwidth allows these signals to pass. The low pass filter bandwidth
determines the bandwidth of detection. Only the signal at the reference frequency will

− ω

noise

nor ω

ref

noise

+ ω

are close to dc). Noise at frequencies very

ref

| is small). Their attenuation depends upon the low pass filter bandwidth

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 39

result in a true dc output and be unaffected by the low pass filter. This is the signal we
want to measure.

Where does the lock-in reference come from?

We need to make the lock-in reference the same as the signal frequency, i.e. ωr = ωL. Not
only do the frequencies have to be the same, the phase between the signals cannot change

with time, otherwise cos(θ

sig

− θ

) will change and V

ref

will not be a dc signal. In other

psd

words, the lock-in reference needs to be phase-locked to the signal reference.

Lock-in amplifiers use a phase-locked loop (PLL) to generate the reference signal. An
external reference signal (in this case, the reference square wave) is provided to the lock­in. The PLL in the lock-in “locks” the internal reference oscillator to this external

reference, resulting in a reference sine wave at ω

with a fixed phase shift of θ

r

. Since

ref

the PLL actively tracks the external reference, changes in the external reference
frequency do not affect the measurement.

All lock-in measurements require a reference signal

In this case, the reference is provided by the excitation source (the function generator).
This is called an external reference source. In many situations, the lock-in’s internal
oscillator may be used instead. The internal oscillator is just like a function generator
(with variable sine output and a TTL sync) which is always phase-locked to the reference
oscillator.

Magnitude and phase

Remember that the PSD output is prop ortional to V
phase difference between the signal and the lock-in reference oscillator. By adjusting θ

we can make θ equal to zero, in which case we can measure V
if θ is 90°, there will be no output at all. A lock-in with a single PSD is called a single-

phase lock-in and its output is V

cosθ.

sig

This phase dependency can be eliminated by adding a second PSD. If the second PSD
multiplies the signal with the reference oscillator shifted by 90°, i.e. sin(ω
its low pass filtered output will be

V

V

= 1/2 V

psd2

~ V

psd2

sinθ

sig

sin(θ

sig

sig

− θ

)

ref

Now we have two outputs, one proportional to cosθ and the other proportional to sinθ. If

we call the first output X and the second Y,

X = V

cosθ Y = V

sig

sig

sinθ

these two quantities represent the signal as a vector relative to the lock-in reference
oscillator. X is called the 'in-phase' component and Y the 'quadrature' component. This is

because when θ = 0, X measures the s ig nal whi le Y is zero.

cosθ where θ = (θ

sig

(cosθ = 1). Conversely,

sig

sig

− θ

t + θ

L

). θ is the

ref

+ 90°),

ref

ref

By computing the magnitude (R) of the signal vector, the phase dependency is removed.

R = (X

SR865 DSP Lock-in Amplifier

2

+ Y2)

1/2

= V

sig

40 Basics Chapter 2

R measures the signal amplitude and does not depend upon the phase between the signal
and lock-in reference.

A dual-phase lock-in, such as the SR865, has two PSD's, with reference oscillators 90°

apart, and can measure X, Y and R directly. In addition, the phase θ between the signal

and lock-in reference, can be measured according to

θ = tan

−1

(Y/X)

What Does a Lock-in Measure?

So what exactly does the SR865 measure? Fourier's theorem basically states that any
input signal can be represented as the sum of many sine waves of differing amplitudes,
frequencies and phases. This is generally considered as representing the signal in the
“frequency domain”. Normal oscilloscopes display the signal in the “time domain”.
Except in the case of clean sine waves, the time domain representation does not convey
very much information about the various frequencies that make up the signal.

What does the SR865 measure?

The SR865 multiplies the input signal by a pure sine wave at the reference frequency. All
components of the input signal are multiplied by the reference simultaneously.
Mathematically speaking, sine waves of differing frequencies are orthogonal, i.e. the
average of the product of two sine waves is zero unless the frequencies are exactly the
same. In the SR865, the product of this multiplication yields a dc output signal
proportional to the component of the signal whose frequency is exactly locked to the
reference frequency. The low pass filter which follows the multiplier provides the
averaging which removes the products of the reference with components at all other
frequencies.

The SR865, because it multiplies the signal with a pure sine wave, measures the single
Fourier (sine) component of the signal at the reference frequency. Let's take a look at an
example. Suppose the input signal is a simple square wave at frequency f. The square
wave is actually composed of many sine waves at multiples of f with carefully related
amplitudes and phases. A 2V pk–pk square wave can be expressed as

S(t) = 1.273 sin(ωt) + 0.4244 sin(3ωt) + 0.2546 sin(5ωt) + …

where ω = 2πf. The SR865, locked to f will single out the first component. The measured
signal will be 1.273 sin(ωt), not the 2V pk–pk that you'd measure on a scope.

In the general case, the input consists of signal plus noise. Noise is represented as varying
signals at all frequencies. The ideal lock-in only responds to noise at the reference
frequency. Noise at other frequencies is removed by the low pass filter following the
multiplier. This “bandwidth narrowing” is the primary advantage that a lock-in amplifier
provides. Only inputs at the reference frequency result in an output.

RMS or Peak?

Lock-in amplifiers as a general rule display the input signal in Volts RMS. When the
SR865 displays a magnitude of 1V (rms), the component of the input signal at the
reference frequency is a sine wave with an amplitude of 1 Vrms or 2.8 V pk–pk.

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 41

A input

Ref in

Timebase

in

Timebase
out

BlazeX
out

10 MHz

crystal Clock

generator

RC

Filter

RC

Filter

Gain/

Offset/

Expand

Annenuator

R, θ

Calc

FIR

Filter

IIR

Filter

Sync

Filter

PLL

Internal

oscillator

Phase

locked

loop

X PSD

Y PSD

10kΩ

50Ω

1MΩ

1kΩ

10Ω

100Ω

B input

I input

Signal
monitor

Gnd/
Float

50Ω/1MΩ

Range

Virtual
ground

x0.1,

x0.3,

...x100

A/A–B

AC/DC couple

Sensitivity

Sine/TTL

Input

Range

Differential
amplifier

Digital signal processor

Reference input

Timebase input

Signal input

Low pass filters

X out

R

X

BlazeX

Sync

Sine

+

–

10 MHz

Y

θ

Sine
out

Y out

Ch 1 out

Ch 2 out

RC

Filter

90°

phase

shift

FIR

Filter

IIR

Filter

Sync

Filter

ADC

ADC

DAC

DAC

DAC

DAC

DAC

DAC

CPU

System

clock

θ

ref

phase

shift

Thus, in the previous example with a 2 V pk–pk square wave input, the SR865 would
detect the first sine component, 1.273 sin(ωt). The measured and displayed magnitude
would be 0.90 V (rms), e.g. 1.273/√2.

Degrees or Radians?

In this discussion, frequencies have been referred to as f (Hz) and ω (2πf radians/sec).
This is because people measure frequencies in cycles per second and math works best in
radians. For purposes of measurement, frequencies as measured in a lock-in amplifier are
in Hz. The equations used to explain the actua l calcu lat ions are sometimes written using
ω to simplify the expressions.

Phase is always reported in degrees. Once again, this is by custom. Equations written as
sin(ωt + θ) are written as if θ is in radians mostly for simplicity. Lock-in amplifiers
always manipulate and measure phase in deg ree s.

Block diagram

A simplified block diagram of the SR865’s lock-in circuit is shown below and explained
in the following sections.

SR865 DSP Lock-in Amplifier

42 Basics Chapter 2

The Reference Oscill ator

A lock-in amplifier requires a reference oscillator phase-locked to the signal frequency.
In general, this is accomplished by phase-locking an internal oscillator to an externally
provided reference signal. Th is refere nce sig nal usua ll y comes from the sign al so urce that
is providing the excitation to the experiment.

Reference Input

The SR865 reference input can trigger on an analog signal (like a sine wave) or a TTL
logic signal. The first case is called External Sine. The input is ac coupled (above 1 Hz)
and the input impedance is 1 MΩ. A sine wave input greater than 200 mV pk is required.
Positive zero crossings are detected and considered to be the zero for the reference phase
shift.

TTL reference signals can be used at all frequencies up to 2.5 MHz. For frequencies
below 1 Hz, a TTL reference signal is required. Many function generators provide a
TTL SYNC output which can be used as the reference. This is convenient since the
generator's sine output might be smaller than 200 mV or be varied in amplitude. The
SYNC signal will provide a stable reference regardless of the sine amplitude.

When using a TTL reference, the reference input trigger can be set to Pos TTL (detect
rising edges) or Neg TTL (detect falling edges). In each case, the internal oscillator is
locked (at zero phase) to the detected edge.

Internal Oscillator

The internal oscillator in the SR865 is basically a 2.5 MHz function generator with sine
and sync outputs. The oscillator generates a digitally synthesized sine wave. The internal
oscillator sine wave is output differentially at the SINE OUT BNC’s on the front panel.
An attenuator sets the amplitude of the output to a value between 1 nV and 2 V (rm s ).

When an external reference is used, this internal oscillator sine wave is phase-locked to
the reference. The rising zero crossing is locked to the detected reference zero crossing or
edge. In this mode, the SINE OUT provides a sine wave phase-locked to the external
reference. Phase locking is accomplished digitally by the SR865.

The internal oscillator may be used without an external reference. In the Internal
Reference mode the frequency is set in the lock-in and the SINE OUT provides the
excitation for the experiment. The phase-locked-loop is not used in this mode.

The BlazeX output on the rear panel can be configured to provide the sync output. The
internal oscillator's rising zero crossings are detected and the output is a square wave.

Reference Oscillators and Phase

The internal oscillator sine wave is not the reference signal to the phase sensitive
detectors. The SR865 computes a second sine wave, phase shifted by θ
internal oscillator (and thus from an external reference), as the reference input to the X

phase sensitive detector. This waveform is sin(ω

t + θ

r

). The reference phase shift is

ref

adjustable in 0.001° increments.

from the

ref

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 43

The input to the Y PSD is a third sine wave, computed by the SR865, shifted by 90° from

the second sine wave. This waveform is sin(ω

t + θ

r

+ 90°).

ref

The phase shifts (θ

and the 90° shift) are exact numbers and accurate to better than

ref

0.000001°. Neither waveform is actually output in analog form since the phase sensitive
detectors are actua lly digital mu ltip li er s inside the SR865.

Phase Jitter

When an external reference is used, the phase-locked loop contributes a little phase jitter.
The internal oscillator is supposed to be locked with zero phase shift relative to the
external reference. Phase jitter means that the average phase shift is zero but the
instantaneous phase shift has a few microdegrees or millidegrees of noise. This shows up
at the output as noise in phase or quadrature measurements.

Phase noise can also cause noise to appear at the X and Y outputs. This is becau se a
reference oscillator with a lot of phase noise is the same as a reference whose frequency
spectrum is spread out. That is, the reference is not a single frequency, but a distribution
of frequencies about the true reference frequency. These spurious frequencies are
attenuated quite a bit but can still cause problems. The spurious reference frequencies
result in signals close to the reference being detected. Noise at nearby frequencies now
appears near dc and affects the lock-in output.

Phase noise in the SR865 is very low and generally causes no problems. In applications
requiring no phase jitter, the internal reference mode should be used. Since there is no
PLL, the internal oscillator and the referen ce sine waves are directly linked and there is
no jitter in the measured phase. (Actually, the phase jitter is the phase noise of a crystal
oscillator and is very, very small).

Harmonic Detection

It is possible to compute the two PSD reference sine waves at a multiple of the internal
oscillator frequency. In this case, the lock-in detects signals at N× f
synchronous with the reference. The SINE OUT frequency is not affected. The SR865
can detect at any harmonic up to N=99 as long as N × f

The Phase Sensitive Detectors

The amplified input signal is converted to digital form using A/D converter sampling at
10 MHz. The SR865 then multiplies the signal with the reference sine wave digitally.

The dynamic reserve is limited by the quality of the A/D conversion. Once the input
signal is digitized, no further errors are introduced. Certainly the accuracy of the
multiplication does not depend on the size of the numbers. The A/D converter used in the
SR865 is extremely linear, meaning that the presence of large noise signals does not
impair its ability to correctly digitize a small signal. In fact, the dynamic reserve of the
SR865 can exceed 120 dB without any problems. We'll talk more about dynamic reserve
a little later.

We've discussed how the digital signal processor in the SR865 computes the internal
oscillator and two reference sine waves and handles both phase sensitive detectors. In the

which are

ref

does not exceed 2.5 MHz.

ref

SR865 DSP Lock-in Amplifier

44 Basics Chapter 2

next section, we'll see how the SR865 performs the low pass filtering and dc
amplification required at the output of the PSD's.

Time Constants and Sensitivity

Remember, the output of the PSD contains many signals. Most of the output signals have
frequencies which are either the sum or difference between an input signal frequency and
the reference frequency. Only the component of the input signal whose frequency is
exactly equal to the reference frequency will result in a dc output.

The low pass filter at the PSD output removes all of the unwanted ac signals, both the 2f
(sum of the signal and the reference) and the noise components. This filter is what makes
the lock-in such a narrow band detector.

RC filters

Traditionally, the time constant setting of a lock-in amplifier determines the bandwidth of
an RC lowpass filter. The time constant is simply 1/2πf where f is the −3 dB frequency of
the filter. The low pass filters are simple 6 dB/octave roll off, RC type filters. A 1 second
time constant referred to a filter whose −3 dB point occurred at 0.16 Hz and rolled off at
6 dB/octave beyond 0.16 Hz. Typically, there are two successive filters so that the overall
filter can roll off at either 6 dB or 12 dB per octave. The time constant referred to the

−3 dB point of each filter alone, not the combined filter.

The notion of time constant arises from the fact that the actual output is supposed to be a
dc signal. In fact, when there is noise at the input, there is noise on the output. By
increasing the time constant, the output becomes more steady and easier to measure
reliably. The tradeoff comes when real changes in the input signal take many time
constants to be reflected at the output. This is because a single RC filter requires about 5
time constants to settle to its final value. The time constant reflects how slowly the output
responds, and thus the degree of output smoothing.

The time constant also determines the equivalent noise bandwidth (ENBW) for noise
measurements. The ENBW is not the filter −3 dB pole, it is the effective bandwidth for
Gaussian noise. More about this later.

The digital signal processing in the SR865 handles all of the low pass filtering. Each PSD
can be followed by up to four filter stages for up to 24 dB/oct of roll off.

Why are multiple filter stages desirable? Consider an example where the reference is at 1
kHz and a large noise signal is at 1.05 kHz. The PSD noise outputs are at 50 Hz
(difference) and 2.05 kHz (sum). Clearly the 50 Hz component is the more difficult to
low pass filter. If the noise signal is 80 dB above the full scale signal and we would like
to measure the signal to 1% (−40 dB), then the 50 Hz component needs to be reduced by
120 dB. To do this in two stages would require a time constant of at least 3 seconds. To
accomplish the same attenuation in four stages only requires 100 ms of time constant. In
the second case, the output will respond 30 times faster and the experiment will take less
time.

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 45

Advanced Filters

The SR865 also provides advanced filtering in place of the traditional 6dB/octave RC
filters. These filters can provide digital filtering that has no analog filter counterpart, and
can settle up to twice as quickly as corresponding RC filters. If the input signal is
changing rapidly, or if there is significant noise away from the reference frequency, then
the advanced filters often provide better signal to noise with faster output response. See
Appendix A for more information.

Floating Point Math in the SR865

The output points of the digital PSD in the SR865 are converted into floating point
numbers. These numbers reflect the actual analog signal gain preceding the analog to
digital converter and are simply the signal input voltages at the input BNC’s. All digital
filtering except for the Sync filter (described below) is performed using floating point
math.

The SR865 plots these floating point outputs in the strip chart displays in units of Volts
(or Amps) referred to the signal inputs. These values have no real limit in size (either too
small or too large) and do not overload.

Sensitivity

So how does the SR865 provide an analog output proportional to the signal when the
result is a floating point value that can range between 10
output can only range between +10V and −10V?

−20

to 1020, while the analog

The answer is that the user must set a Sensitivity which sets the output voltage
corresponding to full scale (10 V) at the output BNC. The Sensitivity also sets the scale
for the displayed bar graphs and numerical readouts of X, Y and R. Note that this is a
numerical output conversion. Output overloads do not affect the actual measurement
results. They only indicate that the output value exceeds 100% of the chosen Sensitivity
and the output BNC, the bar graph and the displayed numerical readout will be pinned at
their maximums. The results displayed on the strip charts or available over the computer
interfaces are the floating point outputs and are unaffected by output overloads.

The Sensitivity is chosen to conveniently and accurately display the measurement results
on the output BNC, the bar graph and the numerical readout. The Sensitivity, however,
must be chosen appropriately when using synchronous filters (see below).

Synchronous Filters

Even if the input signal has no noise, the PSD output always contains a component at 2f
(sum frequency of signal and reference) whose amplitude equals or exceeds the desired
dc output depending upon the phase. At low frequencies, the time constant required to
attenuate the 2f component can be quite long. For example, at 1 Hz, the 2f output is at 2
Hz and to attenuate the 2 Hz by 60 dB in two stages requires a time constant of 3
seconds.

A synchronous filter, on the other hand, operates totally differently. The PSD output is
averaged over a complete cycle of the reference frequency. The result is that all
components at multiples of the reference (2f included) are notched out completely. In the
case of a clean signal, almost no additional filtering would be required. This is
increasingly useful the lower the reference frequency. Imagine what time constant would
be needed at 0.001 Hz!

SR865 DSP Lock-in Amplifier

46 Basics Chapter 2

In the SR865, synchronous filters are av ailabl e at de tection frequencies up to 4.8 kHz. At
higher frequencies, synchronous filters are not required because 2f is easily removed with
the other lowpass filters. The synchronous filter is applied after the other lowpass filters
have removed the noise components from the PSD output leaving, just the synchronous
signals for the synchronous filter to remove. This combination of filters removes all
multiples of the reference frequency and provides overall noise attenuation as well.

Synchronous Filters and Sensitivity

It is important to note that the synchronous filter requires the Sensitivity to be set
appropriately. This is because the synchronous filter is an integer filter and requires the
floating point output to be converted to integer values. This conversion is based upon the
Sensitivity. The output of the synchronous filter is then converted back to floating point.
Choose a Sensitivity as if you will be using the analog X and Y outputs. This will
typically result in the optimum scale. If the Sync filters are overloaded, the Sync error
indicators on the display will light.

Outputs and Scales

The SR865 has X and Y outputs on the rear panel and Channel 1 and 2 (CH1 and CH2)
outputs on the front panel.

X and Y Rear Panel Outputs

The X and Y rear panel outputs are the outputs from the two phase sensitive detectors
with low pass filtering, offset and expand. These outputs are the traditional outputs of an
analog lock-in. The X and Y outputs update at 2.0 MHz.

CH1 and CH2 Front Panel Outputs

The two front panel outputs can be configured to output voltages proportional to X or R
and Y or θ. These outputs update at 2.0 MHz.

If the outputs are set to X or Y, these outputs duplicate the rear panel outputs.

X, Y, R and θ Output Scales

The Sensitivity setting of the SR865 determines what input signal corresponds to 10 V
full scale output for X, Y and R. For example, a Sensitivity of 100 mV means that a
signal at f
maximum of 10 V, and a minimum of –10 V, depending upon the phase of the signal.

The Sensitivity also sets the scale for the displayed bar graphs and numeric readouts.

If the signal input exceeds the sensitivity, the outputs will overload. The actual
measurement is typically unaffected since it is done in floating point and has no overload.
The data displayed in the chart will still be accurate, provided the Sync filter is not
overloaded.

of 100 mVrms will result in a 10 V output of R. X and Y will reach a

ref

Lock-in amplifiers are designed to measure the RMS v alue of the ac input signa l. All
values of X, Y and R outputs and displays are RMS values.

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 47

10VExpand

V 1.000

In Ratio

Offset

ySensitivit

Ror Y, X,

Output ××



































−

=

θ ranges from −180° to +180° regardless of the sensitivity or range. When CH2 outputs a
voltage proportional to θ, the output scale is 18°/Volt or 180°=10 V. The phase bar graph

and numeric readout scales are also unaffected by the Sensitivity.

X, Y and R Output Offset, Ratio and Expand

The SR865 has the ability to offset the X, Y and R outputs. This is useful when
measuring deviations in the signal aroun d som e nom inal value. The offset can be set so
that the output is offset to zero. Changes in the output can then be read directly from the
display or output voltages. The offset is specified as a percentage of the Sensitivity and
the percentage does not change when the Sensitivity is changed. Offsets up to ±999% can
be programmed.

The X, Y, and R outputs may also be rescaled with a ratio function. This allows sig n als to
be normalized to some experimental parameter being monitored with an auxiliary input
voltage. Whenever enabled, ratio is indicated by the
the display.

The X, Y and R outputs may also be expanded. This simply takes the output (minus its
offset) and multiplies by an expansion factor. Thus, a signal which is only 10% of
sensitivity can be expanded to provide 10 V of output rather than only 1 V. The normal
use for expand is to expand the measurement resolution around some value which is not
zero. For example, suppose a signal has a nominal value of 0.9 mV and we want to
measure small deviations, say 10 µV or so, in the signal. The Sensitivity of the lock-in is
set to 1 mV to accommodate the nominal signal. If the offset is set to 90% of full scale,
then the nominal 0.9 mV signal will result in a zero output. The 10 µV deviations in the
signal only provide 100 mV of dc output. If the output is expanded by 10, these small
deviations are magnified by 10 and provide outputs of 1 VDC.

RATIO LED and “Ratio” labels on

The SR865 can expand the output by 10 or 100 provided the expanded output does not
exceed 10 V. In the above example, the 10 µV deviations can be expanded by 100 times
before they exceed full scale (at 1 mV Sensitivity).

The analog output with offset, ratio and expand is

where Offset is a fraction of 1 (50%=0.5), Expand is 1, 10 or 100, and the output cannot
exceed 10 V. When enabled, Ratio In is the voltage on either Aux 3(for X or Y) or Aux 4
(for R); if disabled, the Ratio In factor is simply 1. In the above example (without ratio),

Output = (0.91mV/1mV − 0.9) × 10 × 10V = 1V

for a signal which is 10 µV greater than the 0.9 mV nominal. (Offset = 0.9 and
expand = 10).

SR865 DSP Lock-in Amplifier

48 Basics Chapter 2

The X and Y offset, ratio and expand functions in the SR865 are output functions and do
not affect the calculation of R or θ. R has its own output offset, ratio and expand. θ has no
offset or ratio or expand capability. To offset θ, simply use Auto Phase.

X, Y and R Display Offset and Ratio

Output offsets and ratios are reflected in the displays. For example, when the X output is
offset to zero, the displayed value will drop to zero also. This means that the bar graph
and numeric readout both drop to zero. In addition if X is being charted on the graph, its
graph will drop to zero. Any display which is showing a quantity which is affected by a
non-zero offset will display a highlighted Offset indicator within its display. Similarly,
any display which is showing a quantity affected by a non-unity ratio factor will display a
highlighted Ratio indicator within its display. Remote queries of offset and/or ratioed
quantities are also affected by the offset or ratio.

X, Y and R Display Expand

Output expands do not increase the displayed numeric values of X, Y or R. Expand
increases the resolution of the displayed X, Y or R numeric value. This is because the
expand function increases the resolution of the output, not the size of the input signal.
The displayed value will show an increased resolution but will continue to display the
original value minus the offset. Any display which is showing a quantity which is
affected by a non-unity expand will display a highlighted Expand indicator within its
display.

Output expands affect the bar graphs. The bar graphs are simply a visualization of the
BNC outputs and as such are expanded to provide more visual resolution.

Output expands do not affect the strip charts. The values being charted are already
floating point numbers with all of the resolution available. The strip charts do reflect the
offsets however.

What is Dynamic Reserve Really?

Suppose the lock-in input consists of a signal at f
The real world definition of dynamic reserve is the ratio of the largest noise signal to the
actual signal (at f

). This ratio is usually expressed in dB. For example, if the f

ref

1 µV and the noise reaches 1 mV, then the dynamic reserve is 60 dB (noise is 1000 times
the signal).

Dynamic reserve is therefore a property of the input signal generated by the experiment;
that is, the ratio of noise to signal at the BNC is determined by factors outside the lock-in.
It is the job of the lock-in to measure the signal to the best of its ability, what ev er the
ratio of noise to signal at the input. Of course, it is always better to have less noise and
more signal!

Dynamic Reserve in the SR865

Unlike most lock-ins, the SR865 does not have a dynamic reserve setting. As mentioned
above, the real world dynamic reserve is the noise to signal ratio at the input. The SR865
is designed to achieve the best possible measurement as easily as possible.

plus noise at some other frequencies.

ref

signal is

ref

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 49

In the SR865 the only real gain is in the input amplifier, which amplifies the signal before
it reaches the A/D converter. After this point, the signal is processed digitally so there’s
no further gain. Therefore, the only question is how to set the gain of the input amplifier.

In principle, the gain only needs to be high enough such that the input noise of the signal
and the amplifier is greater than the input noise of the A/D converter. Increasing the
analog gain beyond this point will not reduce the output noise.

Input Range

The Input Range sets the analog gain in the SR865. The settings reflect the largest signal
at the input before the amplifier overloads. Setting the Input Range to a smaller value
increases the analog gain. The signal strength LEDs on the front panel indicate how much
of the A/D range is being used. The general rule is to decrease the Input Range as much
as possible without overloading the amplifier. This increases the A/D range used and
optimizes the output noise performance of the lock-in. At an Input Range of 10 mV no
additional gain is available since noise performance will not improve with more gain.

Remember, we have defined dynamic reserve as a property of the input signal generated
by the experiment. The ratio of noise to signal at the BNC is determined by factors
outside the lock-in. The Input Range must be set to accommodate the largest signal
present at the input, whether it is at f
the Time Constant filters to achieve the best results. Then set the Sensitivity to optimize
the bar graphs and numeric displays as well as the X, Y and R outputs.

or is just noise. Once the Input Range is set, use

ref

SR865 DSP Lock-in Amplifier

50 Basics Chapter 2

The Input Amplifier

A lock-in can measure signals as small as a few nanovolts. A low noise signal amplifier
is required to boost the signal to a level where the A/D converter can digitize the signal
without degrading the signal to noise. The analog gain in the SR865 ranges from roughly
1 to 100. As discussed previously, higher gains do not improve signal to noise and are not
necessary. The gain is set by the Input Range.

What is the Noise Floor?

The input noise of the SR865 signal amplifier is about 2.5 nVrms/√Hz. What does this
noise figure mean? Let's set up an experiment. If the SR865’s Input Range is set to
10 mV then the input gain is sufficient that the noise floor of the measurement is
determined by the input noise. Suppose the PSD output is low pass filtered with a single
RC filter (6 dB/oct roll off) with a time constant of 100 ms. What will be the noise floor
of the measurement?

Amplifier input noise and Johnson noise of resistors are Gaussian in nature. That is, the
amount of noise is proportional to the square root of the bandwidth in which the noise is
measured. A single stage RC filter has an equivalent noise bandwidth (ENBW) of 1/(4T)
where T is the time constant (R×C). This means that Gaussian noise is filtered with an
effective bandwidth equal to the ENBW. In this example, a single 100ms low-pass filter
has an ENBW of 1/(4×100ms) or 2.5 Hz. Thus the lock-in noise floor will be

2.5 nVrms/√Hz × √2.5Hz or ~4 nVrms. For Gaussian noise, the peak to peak noise is
about 5 times the rms noise. Thus, the output will have about 20 nV pk–pk of noise.

Remember that the SR865 reports its measurements in Volts referred to the input BNC.
In this case, the SR865 will appear to have 20 nV pk–pk of noise at f

at the input, with

ref

its input grounded and no signal even applied. If this noise floor is too large for your
experiment, then you need to add more filter stages or increase the time constant to
decrease the ENBW.

All of this assumes that the signal input is being driven from a low impedance source.
Remember resistors have Johnson noise equal to 0.13×√R nVrms/√Hz. Even a 50Ω
resistor has almost 1 nVrms/√Hz of noise. A signal source impedance of 400 Ω will have
a Johnson noise greater than the SR865's input noise. To determine the overall noise of
multiple noise sources, take the square root of the sum of the squares of the individual
noise figures. For example, if a 400 Ω source impedance is used its Johnson noise will be

2.6 nVrms/√Hz. The overall noise at the SR865 input will be [(2.6)

2

+ (2.5)2]

1/2

or

3.6 nVrms/√Hz.

We'll talk more about noise sources later in this section.

At lower gains (Input Ranges above 10 mV), there is not enough gain to amplify the
input noise to a level greater than the noise of the A/D converter. In these cases, the
apparent input noise increases with the Input Range. For example, the configuration
above will appear to have 30 nV pk–pk noise at the input when the Input Range is set to
100 mV and 250 nV pk–pk when the Input Range is 1 V. This means that even with 1 V
of interfering noise, the SR865 can measure f

signals below 1 μV by increasing the

ref

amount of low pass filtering.

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 51

Anti-aliasing Filter

Between the input amplifier and A/D converter there is an anti-aliasing filter. This filter
is required by the signal digitization process. According to the Nyquist criterion, signals
must be sampled at a frequency at least twice the highest signal frequency. In this case,
the highest signal frequency is 2.5 MHz and the sampling frequency is 10 MHz so things
are OK. However, no signals above 5 MHz can be allowed to reach the A/D converter.
These signals would violate the Nyquist criterion and be undersampled. The result of this
undersampling is “aliasing”: these higher frequency signals appe ar as low er freque n cies
in the digital data stream. Thus a signal at 9 MHz would appear as 1 MHz in the digital
data stream and be detectable by the digital PSD. This would be a problem.

To avoid this aliasing, the analog signal is filtered to remove any signals above 2.5 MHz.
This filter has a flat pass band from dc to 3 MHz so as not to affect measurements in the
operating range of the lock-in. The filter rolls off rapidly from 3 MHz to 8 MHz.
Amplitude variations and phase shifts due to this filter are calibrated out at the factory
and do not affect measurements. This filter is transparent to the user.

Input Impedance

The input impedance of the SR865 is 10 MΩ. If a higher input impedance is desired, then
a preamplifier such as the SR550 must be used. The SR550 has an input impedance of
100 MΩ and is ac coupled from 1 Hz to 100 kHz.

Input Connections

In order to achieve the best accuracy for a given measurement, care must be taken to
minimize the various noise sources that can be found in the laboratory. With intrinsic
noise (Johnson noise, 1/f noise or input noise), the experiment or detector must be
designed with these noise sources in mind. These noise sources are present regardless of
the input connections. The effect of noise sources in the laboratory (such as motors,
signal generators, etc.) and the problem of differential grounds between the detector and
the lock-in can be minimized by careful input connections.

There are two basic methods for connecting a voltage signal to the lock-in. The single­ended connection is more convenient, while the differential connection eliminates
spurious pick-up more effectively.

Single-Ended Voltage Connection (A)

In the first method, the lock-in uses the A input in a single-ended mode. The lock-in
detects the signal as the voltage between the center and outer conductors of the A input
only. The lock-in does not force the shield of the A cable to ground, rather it is internally
connected to the lock-in's ground via a resistor. The value of this resistor is selected by

the user. Float uses 10 kΩ and Ground uses 10Ω. This avoids ground loop problems

between the experiment and the lock-in due to differing ground potentials. The lock-in
lets the shield ‘quasi-float’ in order to sense the experiment ground. However, noise
pickup on the shield will appear as noise to the lock-in. This is bad since the input
amplifier cannot reject this noise. Common mode noise, which appears on both the center
and shield, is rejected by the common-mode rejection of the lock-in input, but noise on
only the shield is not rejected at all.

SR865 DSP Lock-in Amplifier

52 Basics Chapter 2

Differential Voltage Connection (A−B)

The second method of connection is the differential mode. The lock-in measures the
voltage difference between the center conductors of the A and B inputs. Both of the
signal connections are shielded from spurious pick-up. Noise pickup on the shields does
not translate into signal noise since the shields are ignored. The shields of A and B are
connected together and can set to Float or Ground as above.

When using two cables, it is important that both cables travel the same path between the
experiment and the lock-in. In particular, there should not be a large loop area enclosed
by the two cables. Large loop areas are susceptible to magnetic pickup.

Common Mode Signals

Common mode signals are those signals which appear equally on both center and shield
(A) or on the center of both A and B (A−B). With either connection scheme, it is
important to minimize both the common mode noise and the common mode signal.
Notice that the signal source is held near ground potential in both illustrations above. If
the signal source floats at a nonzero potential, the signal which appears on both the A and
B inputs will not be perfectly cancelled. The common mode rejection ratio (CMRR)
specifies the degree of cancellation. For low frequencies, the CMRR of 100 dB indicates
that the common mode signal is canceled to 1 part in 10
a 100 mV common mode signal behaves like a 1 µV differential signal! This is especially
bad if the common mode signal is at the reference frequency (this often happens due to

SR865 DSP Lock-in Amplifier

5

. Even with a CMRR of 100 dB,

Chapter 2 Basics 53

ground loops). The CMRR decreases by about 6 dB/octave (20 dB/decade) starting at
around 1 kHz.

ac vs dc Coupling

The signal input can be either ac or dc coupled. The ac coupling passes signals above 160
mHz (0.16 Hz) and attenuates signals at lower frequencies. ac coupling also degrades the
common mode rejection of differential inputs.

At low signal frequencies, dc coupling is required.

A dc signal, if not removed by the ac coupling filter, will multiply with the reference sine
wave and produce a PSD output at the reference frequency (sometimes referred to as 1f).
This signal is not normally present and needs to be removed by the low pass filter. If the
dc component of the signal is large, then this output will be large and require a long time
constant or synchronous filter to remove. ac coupling removes the dc component of the
signal without any sacrifice in signal as long as the frequency is above 160 mHz.

Current Input (I)

The current input on the SR865 uses a separate input BNC. The current input has a gain
of either 10
either 100 Ω (1 μA range) or 1 kΩ (10 nA range). Currents from 3 µA down to 1 fA full
scale can be measured.

6

or 108 Volts/Amp (1 μA or 10 nA range). The input burden resistance is

The impedance of the signal source is the most important factor to consider in deciding
between voltage and current measurements.

For high source impedances, greater than 1 MΩ (1 μA range) or 100 MΩ (10 nA range),
and small currents, use the current input. Its relatively low input impedance greatly
reduces the amplitude and phase errors caused by the cable cap ac itance-source
impedance time constant. The cable capacitance should still be kept small to minimize
the high frequency noise gain of the current preamplifier.

For moderate to low source impedances, or larger currents, the voltage input is preferred.
A small value resistor may be used to shunt the signal current and generate a voltage
signal. The lock-in then measures the voltage across the shunt resistor. Select the resistor
value to keep the shunt voltage small (so it does not affect the source current) while
providing enough signal for the lock-in to measure.

Which current range should you use? The current range determines the input current
noise of the lock-in as well as its measurement bandwidth. Signals far above the input
bandwidth are attenuated by 6 dB/oct. The noise and bandwidth are listed below.

Range Input Noise

Bandwidth
1 μA 130 fA/√Hz 400 kHz
10 nA 13 fA/√Hz 2 kHz

The current to voltage preamplifier is always dc coupled.

SR865 DSP Lock-in Amplifier

54 Basics Chapter 2

fTR4)rms(

noise

∆= kV

nVENBWR13.0)rms(

noise

=V

Intrinsic (Random) Noise Sources

Random noise finds its way into experiments in a variety of ways. Good experimental
design can reduce these noise sources and improve the measurement stability and
accuracy.

There are a variety of intrinsic noise sources which are present in all electronic signals.
These sources are physical in origin.

Johnson Noise

Every resistor generates a noise voltage across its terminals due to thermal fluctuations in
the electron density within the resistor itself. These fluctuations give rise to an open­circuit noise voltage,

−23

where k=Boltzmann's constant (1.38x10
300 K), R is the resistance in ohms, and ∆f is the bandwidth in hertz. ∆f is the equivalent
noise bandwidth of the measurement.

J/K), T is the temperature in kelvins (typically

Since the input signal amplifier in the SR865 has a bandwidth of approximately 3 MHz,
the effective noise at the amplifier input is V

= 220√R nVrms or 1.1√R μV pk–pk.

noise

This noise is broadband and if the source impedance of the signal is large, can determine
the required Input Range of the lock-in.

The amount of noise measured by the lock-in is determined by the measurement
bandwidth. Remember, the lock-in does not narrow its detection bandwidth until after the
phase sensitive detectors. In a lock-in, the equivalent noise bandwidth (ENBW) of the
low pass filter (time constant) sets the detection bandwidth. In this case, the measured
noise of a resistor at the lock-in input, typically the source impedance of the signal, is
simply

where R is in ohms and ENBW is in hertz. The ENBW is determined by the time
constant (T) and slope as shown below (for normal RC type filters).

Slope

ENBW
6 dB/oct 1/(4T)
12 dB/oct 1/(8T)
18 dB/oct 3/(32T)
24 dB/oct 5/(64T)

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 55

fI2)rms(

noise

∆= qI

The signal amplifier bandwidth determines the amount of broadband noise that will be
amplified. This affects the Input Range (analog signal gain). The time constant sets the
amount of post-demodulation noise which will be measured at the reference frequency.

1/f Noise

Every 10 Ω resistor, no matter what it is made of, has the same Johnson noise. However,

there is excess noise in addition to Johnson noise which arises from fluctuations in
resistance due to the current flowing through the resistor. For carbon composition
resistors, th is is typically 0.1 µV-3 µV of rms noise per Volt of applied across the
resistor. Metal film and wire-wound resistors have about 10 times less noise. This noise
has a 1/f spectrum and makes measurements at low frequencies more difficult.

Other sources of 1/f noise include noise found in vacuum tubes and semiconductors.

Shot Noise

Electric current has noise due to the finite nature of the charge carriers. There is always
some non-uniformity in the electron flow which generates noise in the current. This noise
is called shot noise. This can appear as voltage noise when current is passed through a
resistor, or as noise in a current measurement. The shot noise or current noise is given by

where q is the electron charg e (1.6×10
depending upon the circuit, and ∆f is the bandwidth.

When the current input of a lock-in is used to measure an ac signal current, the bandwidth
is typically so small that shot noise is not important.

Total Noise

All of these noise sources are incoherent. The total random noise is the square root of the
sum of the squares of all the incoherent noise sources.

External Noise Sources

In addition to the intrinsic noise sources discussed previously, there are a variety of
external noise sources within the laboratory.

Most of these noise sources are asynchronous, i.e. they are not related to the reference
and do not occur at the reference frequency or its harmonics. Examples include lighting
fixtures, motors, cooling units, radios, computer screens, etc. These noise source s aff ect
the measurement by increasing the required dynamic reserve or lengthening the time
constant.

−19

coulomb), I is the RMS ac current or dc current

Some noise sources, however, are related to the reference and, if picked up in the signal,
will add or subtract from the actual signal and cause errors in the measu rem en t. Ty pical
sources of synchronous noise are ground loops between the experiment, detector and
lock-in, and electronic pick up from the reference oscillator or experimental apparatus.

SR865 DSP Lock-in Amplifier

56 Basics Chapter 2

noisestraystray

VC

V

C

ω

==

dt

d

i

Many of these noise sources can be minimized with good laboratory practice and
experiment design. There are several ways in which noise sources are coupled into the
signal path.

Capacitive coupling

An ac voltage from a nearby piece of apparatus can couple to a detector via a stray
capacitance. Although C
a weak experimental signal. This is especially troublesome if the coupled noise is
synchronous (at the reference frequ ency).

may be very small, the coupled noise may still be larger than

stray

We can estimate the noise current caused by a stray capacitance by:

where ω is 2π times the noise frequency, V

is the noise amplitude, and C

noise

stray

is the

stray capacitance.

For example, if the noise source is a power circuit, then f = 60 Hz and V
C

can be estimated using a parallel plate equivalent capacitor. If the capacitance is

stray

roughly an area of 1 cm

2

separated by 10 cm, then C

is 0.009 pF. The resulting noise

stray

= 120 V.

noise

current will be 400 pA (at 60 Hz). This small noise current can be thousands of times
larger than the signal current. If the noise source is at a higher frequency, the coupled
noise will be even greater.

If the noise source is at the reference frequency, then the problem is much worse. The
lock-in rejects noise at other frequencies, but pick-up at the reference frequency appears
as signal!

Cures for capacitive noise coupling include:

1) Removing or turning off the noise source.

2) Keeping the noise source far from the experiment (reducing C

). Do not bring

stray

the signal cables close to the noise source.

3) Designing the experiment to measure voltages with low impedance (noise current

generates very little voltage).

4) Installing capacitive shielding by placing both the experiment and detector in a

grounded metal box.

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 57

Inductive coupling

An ac current in a nearby piece of apparatus can couple to the experiment via a magnetic
field. A changing current in a nearby circuit gives rise to a changing magnetic field which
induces an emf (dØ
a transformer with the experiment–detector loop as the secondary winding.

Cures for inductively coupled noise include:

1) Removing or turning off the interfering noise source.

2) Reduce the are a of the pick -up loop by using twisted pairs or coaxial cables, or
even twisting the 2 coaxial cables used in differential connections.

/dt) in the loop connecting the detector to the experiment. This is like

B

3) Using magnetic shielding to prevent the magnetic field from crossing the area of
the experiment.

4) Measuring currents, not voltages, from high impedance detectors.

Resistive coupling or ground loops

Currents flowing through the ground connections can give rise to noise voltages. This is
especially a problem with reference frequency ground currents.

In this illustration, the detector is measuring the signal relative to a ground far from the
rest of the experiment. The experiment senses the detector signal plus the voltage due to
the noise source's ground return current passing through the finite resistance of the
ground between the experiment and the detector. The detector and the experiment are
grounded at different places which, in this case, are at different potentials.

Cures for ground loop problems include:

1) Grounding everything to the same physical point.

SR865 DSP Lock-in Amplifier

58 Basics Chapter 2

i

dt

d

dt

d

dt

d

==+

QC

V

V

C

2) Using a heavy ground bus to reduce the resistance of ground connections.

3) Removing sources of large ground currents from the ground bus used for small
signals.

4) Using differential excitation and signal sensing.

Microphonics

Not all sources of noise are electrical in origin. Mechanical noise can be translated into
electrical noise by microphonic effects. Physical changes in the experiment or cables (due
to vibrations for example) can result in electrical noise over the entire frequency range of
the lock-in.

For example, consider a coaxial cable connecting a detector to a lock-in. The capacitance
of the cable is a function of its geometry. Mechanical vibrations in the cable translate into
a capacitance that varies in time, typically at the vibration frequency. Since the cable is
governed by Q=CV, taking the derivative, we have

Mechanical vibrations in the cable which cause a dC/dt will give rise to a current in the
cable. This current affects the detector and the m easu re d signal.

Some ways to minimize microphonic signals are:

1) Eliminate mechanic al vib ra tion s near the expe rim ent.

2) Tie down cables carrying sensitive signals so they do not move.

3) Use a low noise cable that is designed to reduce microphonic effects.

Thermocouple effects

The emf created by junctions between dissimilar metals can give rise to many microvolts
of slowly varying potentials. This source of noise is typically at very low frequency since
the temperature of the detector and experiment generally changes slowly. This effect is
large on the scale of many detector outputs and can be a problem for low frequency
measurements, especially in the mHz range.

Some ways to minimize thermocouple effects are:

1) Hold the temperature of the experiment or detector constant.

2) Use a compensation junction, i.e. a second junction in reverse polarity which
generates an emf to cancel the thermal potential of the first junction. This second
junction should be held at the same temperature as the first junction.

Noise Measurements

Lock-in amplifiers can be used to measure noise. Noise measurements are generally used
to characterize components and detectors.

SR865 DSP Lock-in Amplifier

Chapter 2 Basics 59

The SR865 measures input signal noise AT the reference frequency. Many noise sources
have a frequency dependence which the lock-in can measure.

How Is Noise Measured?

Remember that the lock-in detects signals close to the reference frequency. How close?
Input signals within the detection bandwidth set by the low pass filter time constant and
roll-off appear at the output at a frequency f = f
noise at the output with a bandwidth of dc to the detection bandwidth.

For Gaussian noise, the equivalent noise bandwidth (ENBW) of a low pass filter is the
bandwidth of the perfect rectangular filter which passes the same amount of noise as the
real filter.

The ENBW is determined by the time constant (T) and slope as shown below (for normal
RC type filters).

− f

. Input noise near f

sig

ref

appears as

ref

Slope

ENBW
6 dB/oct 1/(4T)
12 dB/oct 1/(8T)
18 dB/oct 3/(32T)
24 dB/oct 5/(64T)

The noise is simply the standard deviation (root of the mean of the squared deviations) of
the measured X or Y values averaged over a period of time. This involves computing the
average of (X

value

− X

)2 where X

mean

is the current X output and X

value

is the mean X

mean

output. The noise result is the square root of the average. The averaging time is roughly
200 time constants; for example, if the time constant is 100 mS, the noise measurement
would take about 20 seconds to settle.

Shorter averaging times yield a very poor estimate of the noise (the mean varies rapidly
and the deviations are not averaged well). Longer averaging times, while yielding better
results, take a long time to settle to a steady answer.

Noise and Sensitivity

The noise is calculated from the final output values of X and Y. In order to accurately
compute the noise, the Sensitivity must be set appropriately so that the displayed values
of X or Y have enough resolution. If the displayed values of X or Y are unchanging, then
the computed noise will be almost zero.

If there is signal at f
The resolution can then be increased using Expand.

, then the mean value of X or Y should be removed using an Offset.

ref

Since the noise measurements are calculated from the scaled output values, they are also
modified by the ratio function if enabled.

Noise Display

To display a noise measurement, choose X or Y noise (Xn or Yn) as one of the displayed
parameters in the [Config] screen. The SR865 is calculating the noise all of the time,
whether or not X or Y noise are being displayed. Thus, as soon as noise is displayed, the
value shown is up to date and no settling time is required. If the time constant or output
scaling is changed, then the noise measurement will need to settle to the new value.

SR865 DSP Lock-in Amplifier

60 Basics Chapter 2

X and Y noise are displayed in units of V(rms). The ENBW of the time constant is NOT
factored into the calculation. To convert to spectral noise density, divide the reading by
√ENBW.

SR865 DSP Lock-in Amplifier

Operation 61

Chapter 3

Operation

Introduction

Power

The power switch is on the rear panel. The SR865 is turned on by pushing the switch up.

Keys

The keys are grouped and labeled according to function. This manual will refer to a key
with square brackets such as [Key]. A complete description of the keys follows in this
section.

Knobs

Knobs are used to adjust the internal refe renc e frequ en cy , referenc e phase sh ift, si ne
output amplitude, sine dc level, offsets and Aux Output levels. The knobs also have
secondary key-press functions labeled in italics below the knobs; these funct ions are
accessed by pressing the knob inward. This manual will refer to knobs in angled brackets
such as <Knob>.

SR865 DSP Lock-in Amplifier

62 Operation Chapter 3

Touchscreen Buttons

Touchscreen buttons on the display allow for direct keypad entry, text entry, calculator
functions, graph scaling and much more. This manual will refer to touchscreen buttons
with curly brackets such as {Button}.

Reset

To reset the unit, hold down the [Local] key while the power is turned on. The unit will
use the standard settings. A similar reset is available without cycling power by pressing
the [Save Recall] key and selecting {Recall default}.

Serial Number and Firmw are V er si on

Press and hold [Local] and then [Auto Range] at the same time to display the unit ser ial
number and version numbers. Press [Screen Layout] to dismiss the informational screen.

Local Lockout

If a computer interface has placed the unit in the Local Lockout state, indicated by the

Remote LED, then the keys, knobs and touchscreen are disabled. Attempts to change the

settings from the front panel will display a message indicating local control is locked out
by the interface.

Reference Input

The reference input can be a sine wave (rising zero crossing detected) or a TTL pulse or
square wave (rising or falling edge). The input impedance is either 1 MΩ or 50 Ω. The
Sine Trig is ac coupled (>1 Hz). For low frequencies (<1 Hz), it is necessary to use a
TTL reference signal. The TTL input provides the best overall performance and should
be used whenever possible.

Sine Out

The internal oscillator outputs are differential with 50 Ω output impedance each. The
output amplitude is specified as Vrms differential into 50 Ω loads (amplitude = sine+ −
sine−). If the output is terminated in a high impedance, the amplitude will be double. If
only a single output is used, the amplitude will be half. Thus a single output into high
impedance will actually have the specified amplitude. The amplitude can be set from
1 nVrms to 2 Vrms.

The dc level of the sine output can also be specified. The dc can be applied differentially
(outputs move apart) or in common (outputs move together). The dc level can be set from

0.1 mV to 5 V. The maximum output is about ±6 V.

These outputs are active even when an external reference is used. In this case, the sine
wave is phase locked to the reference and its amplitud e is programmable.

A 2.5 V sync logic output is provided on the rear panel via the configurable BlazeX
output. This output is useful for triggering scopes and other equipment at the reference
frequency. The sync output is a square wave derived from the zero crossings of the sine
output.

SR865 DSP Lock-in Amplifier

Chapter 3 Operation 63

CH1 & CH2 Outputs

The Channel 1 and Channel 2 outputs can be configured to output a voltage from −10 V
to +10 V proportional to X or R (CH1) and Y or θ (CH2). The sensitivity setting
determines full scale (±10 V).

Signal Inputs

The voltage input mode may be single-ended, A, or differential, A−B. The A and B
inputs are voltage inputs with 10 MΩ, 25 pF input impedance. Their connector shields

are isolated from the chassis by 10 Ω (Ground) or 1 kΩ (Float). Do not apply more than
10 V to either input. The shields should never exceed 1 V.

The current input is configured for either 1 µA or 10 nA current range. The input burden
resistance is 100 Ω (1 µA) or 1 kΩ (10 nA) to a virtual ground.

Key Click On/Off

Press and hold the [Calc/system] key to display the system menu. Touch {Sounds} to
toggle the key click on and off.

Display Off Operation

Press the [Blank] key to operate with the front panel display and LEDs off. The SR865 is
still operating, the outputs are active, data collection continues and the unit responds to
interface commands. To change a setting, press [Blank] to return to normal operation,
change the desired parameter, then press [Blank] again.

Front Panel Test

To test the front panel, press and hold [Local] and [Auto Range] at the same time.

Turn any knob to run through all of the LEDs. Make sure all of the knobs and all of the
LEDs work.

Touch and drag on the touchscreen to display a little trail. The bright end signifies the lift
location. This verifies the touch functionality.

Press each key to toggle its position (green/gray) on the screen. Keys with a secondary
press and hold function need to be held to toggle. The knobs also have a press and hold
feature. Check to see that all keys and knobs function.

The [Screen Layout] key exits this screen.

SR865 DSP Lock-in Amplifier

64 Operation Chapter 3

Standard Settings

If the [Local] key is held down when the power is turned on, the lock-in settings will be set to the defaults
shown below rather than the settings that were in effect when the power was last turned off. The default
settings may also be recalled using the *RST command over the computer interface, or by pressing [Save
Recall] and selecting {Recall default}. In this case, the communications parameters and status registers
are not changed.

Reference

Phase 0.000°
Reference Source Internal
Internal Frequency 100.000 kHz
Harmonic # 1
Sine Amplitude 0.00 Vrms
DC Level 0.0 V
Ext Reference Trigger Sine
Ext Reference Impedance 50 Ω

Signal

Input Voltage
A−B A
Couple AC
Ground Float
Current Range 1 μA
Input Range 1 V
Time Constant 100 ms
Filter Slope 6 dB
Advanced Filter Off
Synchronous Off
Sensitivity 1 V

System/General

10 Mhz Timebase Auto
Sync/BlazeX Output Sync
Sounds On

System/Files

Prefix SR865
Suffix 1
Print Mode Screen
Data Format CSV

Outputs

CH1 Output X
CH2 Output Y
All Offsets 0.00%
All Expands 1
All Ratios None
All Aux Outputs 0.000 V

Display

Green Channel X, Graph On
Blue Channel Y, Graph On
Yellow Channel R, Graph On
Orange Channel θ, Graph On
Screen Layout Trend
Blank No

SR865 DSP Lock-in Amplifier

Operation 65

Signal Input

Input [Select]

The Input [Select] key selects between the voltage and current preamps.

The voltage inputs (A and B) have a 10 MΩ, 25 pF input impedance. Their connector

shields are isolated from the chassis by either 10 Ω (Ground) or 10 kΩ (Float). Do not
apply more than 10 V to either input. The shield s shoul d never exceed 1 V.

The current input uses the I connector. The input burden resistance is 100 Ω (1 µA range)
or 1 kΩ (10 nA range) to a virtual ground. The largest allowable current before overload
is around 3 µA (1 µA range) or 30 nA (10 nA range). No current larger than 10 mA
should ever be applied to this input.

Voltage [A−B]

The voltage input can be either a single-ended (A) or diffe rential (A−B) voltage. The
shields of A and B are connected and grounded by either 10 Ω (Ground) or 10 kΩ (Float).

Voltage [Couple]

This key selects the voltage input coupling. The signal input can be either ac or dc
coupled. The ac coupling option uses a high pass filter to pass signals abov e 160 mHz
and attenuate signals at lower frequencies. ac coupling should be used at frequencies
above 160 mHz whenever possible. At lower frequencies, dc coupling is required. ac

SR865 DSP Lock-in Amplifier

66 Operation Chapter 3

coupling results in gain and phase errors at low frequencies as wells as reduced CMRR
performance.

Remember, the Reference Input is ac coupled when a sine refere nce is used. This a lso
results in phase errors at low frequencies.

Voltage [Ground]

This key chooses the shield grounding configuration. The shields of the input connecto rs
(A and B) are not connected directly to the lock-in chassis ground. In Float mode, the
shields are connected by 10 kΩ to the chassis ground. In Ground mode, the shields are
connected by 10 Ω to ground. Typically, the shields should be grounded if the signal
source is floating and floating if the signal source is grounded. Do not exceed 1 V on the
shields.

Voltage [Input Range]

The voltage input range specifies the largest input signal (ac or dc coupled) before the
voltage input is overloaded. The smaller the input range value, the higher the amplifier
gain leading to the A/D converter. The signal strength LEDs indicate how much of the
A/D range is being used. When the lowest yellow LED is on, try decreasing the input
range (increasing the gain).

In general, use the smallest input range possible without overload. Remember, the largest
signal, whether it’s at f

This setting has no effect when the current input is selected.

The Signal Monitor on the rear panel is the amplifier outpu t.

Current [Range]

The current input uses the I connector. The input burden resistance is 1 kΩ (10 nA range)
or 100 Ω (1 μA range) to a virtual ground. The shield is chassis ground. The largest
allowable current (ac plus dc) before overload is approximately ±4 µA or ±40 nA. No
current larger than 10 mA should ever be applied to this input.

The current range determines the input current noise as well as the input bandwidth. The
10 nA range has 10 times lower noise but 200 times lower bandwidth. Be sure the signal
frequency is below the input bandwidth limit. The noise and bandwidth are listed below.

1 μA 130 fA/√Hz 400 kHz
10 nA 13 fA/√Hz 2 kHz

The impedance of the current source should be greater than 1 MΩ when using the 1 μA
range and greater than 100 MΩ when using the 10 nA range.

Range Noise

or just noise, will be the first to overload.

ref

Bandwidth

The signal strength LEDs indicate how much of the A/D range is being used. The Signal
Monitor on the rear panel is the amplifier output.

The current range setting has no effect when the voltage input is selected.

SR865 DSP Lock-in Amplifier

Chapter 3 Operation 67

Input Overload

The Overload LED in this section indicates an input overload. This occurs for voltage
inputs greater than the voltage input range (unless removed by ac coupling) or current
inputs exceeding ±4 µA (1 µA range) or ±40 nA (10 nA range).

An input overload compromises the measurement of X, Y, R and θ. Any display of these

quantities will be accompanied by an Ovld indication.

Either increase the input range or reduce the input signals to remove the overload.

Slew Rate

Overloads can also be caused by high slew rate voltage signals. These are large signals
with fast rise or fa ll times. This can be a square wave (at any frequency) with fast
transitions, or simply a large amplitude high frequency sine wave. In these cases, the
amplifier may become slew rate limited and unable to accurately amplify other
components of the signal. When this occurs, the
the signal strength indicator is not at its maximum. Once again, increase the input range
or reduce the input signal.

Overload LED may light even though

[Time Constant]

The time constant may be set from 1 µs to 30 ks. The time constant is indicated by 1 or 3
times 1, 10 or 100 with the appropriate units.

This time constant sets the bandwidth of the low pass filter after the phase sensitive
detectors for X and Y. This is the filter that removes sig nals at frequenci es othe r than f
In general, longer time constants provide more noise filtering and quieter measurements
but longer response times.

The time constant also determines the equivalent noise bandwidth (ENBW) of the low
pass filter. This is the measurement bandwidth for X and Y noise and depends upon the
time constant and filter slope. (See the Noise discussion in the Basics section.)

In some experiments, output latency (delay from signal input to analog output) at short
time constants is important. Use the rear panel BlazeX output for the lowest latency
analog X output. Otherwise, the front panel outputs CH1 and CH2 as well as the rear
panel X and Y outputs have sufficient bandwidth for all time constants.

Filter [Slope/adv]

This key selects the number of poles in the low pass filter. Choosing 6, 12, 18 or 24
dB(/oct) selects 1, 2, 3 or 4 poles. Using more poles can decrease the required time
constant and make a measurement faster.

.

ref

The normal low pass filter is an RC filter. This is equivalent to the traditional filter found
in analog lock-ins.

To use advanced filters in place of the RC filters press and hold the [Slope/adv] key until
the

Advanced LED turns on. Brief presses of [Slope/adv] cycles the number of poles

SR865 DSP Lock-in Amplifier

68 Operation Chapter 3

from 1 to 4 (6 to 24 dB). Another press and hold of the [Slope/adv] key reverts the filters
back to RC filters and turns off the

For two filters with the same noise bandwidth, i.e. whose outputs would be equally noisy
if the input was white noise, the advanced filters have faster transient response, less
overshoot, and higher stopband attenuation to ensure greater suppression of out-of-band
spurs.

When the time constant is 3 s or faster an advanced FIR filter pole of equivalent noise
bandwidth (ENBW) is substituted for each RC filter pole.

For time constants of 10 s through 1000 s, the advanced filter is a Linear Phase (LP) filter
with same attenuation slope as the RC filters of the same time constant and number of
poles. In other words, the Linear Phase filter’s stop band is aligned with the
corresponding RC filter.

For time constants greater than 1000 s, the advanced filter is the same as the RC filter.

In an actual experimental situation, the signal is seldom simple. If the goal is to read a
static output value, then longer time constants will achieve that. If the output value is
changing because of parameter sweeping or a signal turning on and off, then the
advanced filters can be of great help. In practice it is simple to try these filters, at various
time constants and number of poles, in comparison with RC filters. They often yield
better results in less time.

Advanced LED.

See Appendix A for more information about the advanced filters.

Filter [Sync]

Pressing this key turns synchronous filtering on or off.

Synchronous filtering removes outputs at harmonics of the reference frequency, most
commonly 2×f
would require very long time constants to remove. The synchronous filter does not
attenuate broadband noise very well. The low pass filters should be used to remove
outputs due to noise and other non-harmonic interfering signals.

The synchronous filter computes moving averages of the X and Y outputs over a
complete reference period, 1/f
zero and are removed. Thus frequency components in the output at n×f
The synchronous filter follows the normal low pass filters (time constant) and is applied
at the outputs.

Sensitivity

In order for the synchronous filter to perform accurately, the sensitivity must be set
appropriately. This is because the synchronous filter acts on the output scaled values of
X and Y. If the sensitivity is set too high, the values of X and Y (in the numeric displays)
will lack the necessary resolution. If the sensitivity is set too low, the values will
overload. In general, setting the sensitivity to display a reasonable amount of bar graph is
sufficient for accurate synchronous filtering. If the synchronous filter overloads because
of the sensitivity, the Sync warning is displayed with displays of X, Y, R or θ.

. This is very effective at low reference frequencies since 2×f

ref

. In this way, outputs with periods 1/(n×f

ref

ref

outputs

ref

) average to

ref

are all removed.

SR865 DSP Lock-in Amplifier

Chapter 3 Operation 69

Internal, External and Chop Reference

When the reference source is internal, external or chop, the synchronous filter period is
1/f

regardless of the harmonic detect number. The output is averaged over N periods of

ref

the detect frequency where N is the harmonic detect number. Synchronous filtering is
only active when the reference frequency is 4.8 kHz or below. If the reference frequency
is above 4.8 kHz and synchronous filtering is on, then a Sync error is displayed with
displays of X, Y, R or θ indicating that the synchronous filter is not working as expected.

Dual Reference

When the reference source is dual, the synchrono us fi lter peri od is a single period of the
actual detect frequency f

= | N1×f

dual

– N2×f

int

|, where N1 and N2 are the internal

ext

harmonic and dual reference external harmonic detect numbers respectively.
Synchronous filtering is only active when the actual detect frequency is 4.8 kHz or
below. If the detect frequency is above 4.8 kHz and synchronous filtering is on, then a
Sync error is displayed with displays of X, Y, R or θ indicating that the synchronous filter
is not working as expected.

Settling Time

When the synchronous filter is active, the phase sensitive detectors (PSD's) are followed
by the specified low pass filtering (time constant filter) and then the synchronous filter.
This removes non-harmonic noise before the synchronous filter.

The settling time of the synchronous filter is one period of the filter (usually 1/f
the synchronous filter follows the phase sensitive detectors, the time constant filters and
output scaling, any change in the signal amplitude, reference frequency, phase, time
constant, slope or sensitivity will cause the outputs to settle for one period of the filter.
These transients are because the synch ron ous fi lter provides a steady output only if its
input is repetitive from period to period.

The transient response also depends upon the time constants of the regular filters. Very
short time constants (<< period) have little effect on the transient response. Longer time
constants (< period) can magnify the amplitude of a transient. Much longer time
constants (≥ period) will increase the settling time far beyond a period.

[Sensitivity]

The Sensitivity setting of the SR865 determines what input signal corresponds to 10 V
full scale output for X, Y and R. For example, a Sensitivity of 100 mV means that a
signal at f
maximum of 10 V depending upon the phase of the signal.

The Sensitivity also sets the scale for the displayed bar graphs and numeric readouts.

Note that this is a numerical output conversion. Output overloads do not generally affect
the actual measurement results! They only indicate that the output value exceeds 100% of
the chosen Sensitivity and the output BNC, the bar graph and the displayed numerical
readout will be pinned at their maximums. The results displayed on the strip charts or
available over the computer interfaces are the floating point outputs and are unaffected by
output overloads. Output overloads are indicated by a ‘Scale’ indicator in the X, Y or R
displays.

of 100 mVrms will result in a 10 V output of R. X and Y will reach a

ref

). Since

ref

SR865 DSP Lock-in Amplifier

70 Operation Chapter 3

The Sensitivity should be chosen to conveniently and accurately display the measurement
results on the output BNC, the bar graph and the numerical readout. The Sync filter also
uses the Sensitivity as an indicator of the signal range it needs to accommodate. An
output overload does not necessarily mean a Sync filter overload, but a ‘Sync’ error
indicated in the X, Y, R or θ displays does signify a Sync filter overload.

θ ranges from −180° to +180° regardless of the scale or range. When CH2 outputs a
voltage proportional to θ, the output scale is 18°/Volt or 180°=10 V. The phase bar graph

and numeric readout scales are also unaffected by the Sensitivity.

Synchronous Filters and Noise Calculations

Synchronous filtering and calculation of X
scaled values of X and Y. Thus, their accuracy is affected by a poor choice of Sensitivity.
If the Sensitivity is set to o large, the values of X and Y (in the numeric displays) will lack
the necessary resolution. If the Sensitivity is set too low, the values will be overloaded
(pinned) and no noise will be measured. In general, setting the Sensitivity to display a
reasonable amount of bar graph is sufficient.

noise

and Y

are performed on the output

noise

SR865 DSP Lock-in Amplifier

Chapter 3 Operation 71

V10Expand

V 1.000

In Ratio

Offset

ySensitivit

Ror YX,

Output ××



































−

=

CH1 and CH2 Outputs: Offset, Ratio and Expand

Analog Outputs

Analog voltages proportional to X, Y, R and θ can be output from the SR865. The X and

Y rear panel outputs always output X and Y. The CH1 output voltage is proportional to

either X or R. The CH2 output voltage is proportional to either Y or θ. The output

voltages are determined by

The output is normally 10 V for an input signal equa l to the sensitivity. The offset
subtracts a percentage of full scale (up to ±999%) from the output. Expand multiplies the
difference by a factor of 1, 10 or 100. This result may be divided by a ratio input (Aux
Input voltage). An Aux Input voltage of 1.000 V corre sponds to unity.

Outputs which would exceed ±10 V generate an Output Overload and the red output

Overload LED will light. Any corresponding displayed numeric value/bar graph will

indicate ‘Scale’ in the display. The actual measurement is unaffected since it is done in
floating point and has no overload. Data displayed in the strip chart will still be accurate.

The X and Y offset, expand and ratio functions in the SR865 are output functions, they
do not affect the calculation of R or θ. R has its own output offset, expand and ratio.

Phase ranges from −180° to +180° regardless of the sensitivity. When CH2 outputs a
voltage proportional to θ, the output scale is 18°/Volt or 180° = 10 V. The phase bar
graph and numeric readout scales are also unaffected by the Sensitivity. Phase has no
offset, ratio or expand capability. To offset phase, simply use Auto Phase. To expand
phase, expand the value of Y in quadrature.

SR865 DSP Lock-in Amplifier

72 Operation Chapter 3

Offset

Output offsets are reflected in the displays. For example, when the X output is offset to
zero, the displayed value will drop to zero also. This means that the bar graph and
numeric readout both drop to zero. In addition if X is being charted on the graph, its
graph will drop to zero. Any display which is showing a quantity which is affected by a
non-zero offset will display a highlighted Offset indicator within its display.

Ratio

The X, Y, and R may be normalized to an Aux Input voltage. This is called a ratio
measurement. An output ratio is reflected in the displays. The numeric values, bar graphs
and strip charts all reflect the normalized output. Any display which is showing a
quantity which is affected by a ratio will display a highlighted Ratio indicator within its
display.

Expand

Output expands do not increase the displayed numeric values of X, Y or R. Expand
increases the resolution of the displayed X, Y or R numeric value. This is because the
expand function increases the resolution of the output, not the size of the input signal.
The displayed value will show an increased resolution but will continue to display the
original value minus the offset. Any display which is showing a quantity which is
affected by a non-unity expand will display a highlighted Expand indicator within its
display.

Output expands affect the bar graphs. The bar graphs are simply a reflection of the BNC
outputs and as such are expanded to provide more visual resolution.

Output expands do not affect the strip charts. The values being charted are already
floating point numbers with all of the resolution available. The strip charts do reflect the
offsets however.

See the SR865 Basics section for more information.

Output [Select]

This key selects the source for the CH1 (CH2) output BNC. CH1 can select either X or R.

CH2 can select either Y or θ. This also determines which parameter’s offset and expand

are adjusted with the [Expand] key and the <Offset> knob.

Output [Expand]

Pressing this key selects the X or R Expand (CH1) or the Y Expand (CH2). Use the
[Select] key to select either X or R (CH1) and Y or θ (CH2). θ cannot be expanded. To
expand phase, expand the value of Y in quadrature.

The expand can be none (×1 or no expand), ×10 or ×100. If the expand is ×10 or ×100,
the corresponding LED will light. The output can never exceed full scale (±10 V) when
expanded. An output expanded beyond full scale will be output overloaded.

SR865 DSP Lock-in Amplifier

Chapter 3 Operation 73

Output <Offset>

Press this knob briefly to to ggle the X or R offset (CH1) or the Y offset (CH2) on and off.
Use the [Select] key to select either X or R (CH1) and Y or θ (CH2). The value of θ
cannot be offset. Use Auto Phase to offset phase to zero.

This allows the offset to be turned on and off without adjusting the actual offset
percentage.

The

OFFSET LED is on when the offset is being applied.

Press and hold the <Offset> knob to display an offset keypad as shown below.

Touch {Auto} Offset to automatically adjust the X offset (or Y or R) such that X (or Y or
R) becomes zero. Offsets can also be set using the keypad or by turning the <Offset>
knob. Turning the knob ALWAYS adjusts the offset, even if the offset is toggled off and
the keypad is not shown.

Offsets are set as a percentage of the Sensitivity up to ±999% (10×). The offset
percentage is not changed with the sensitivity — i t is an output function.

Touch {Close} to dismiss the keypad.

Touch {Ratio} to cycle between Off (no ratio) and an Aux Input. The
when a ratio is being applied.

Output Overload

Outputs which would exceed ±10 V generate an Output Overload and the red Overload
LED will light. Any corresponding displayed numeric value/bar graph will indicate
‘Scale’ in the dis play. The actual measurement is unaffected since it is done in floating
point and has no overload. Data displayed in the strip chart will still be accurate.

RATIO LED is on

SR865 DSP Lock-in Amplifier

74 Operation Chapter 3

Reference

Each of the data screens always displays a reference info bar across the top.

This bar always shows tiles displaying the reference phase, frequency, detected harmonic,
sine out amplitude and dc level. Each of these parameters can be adjusted using the knobs
and buttons in this section of the front panel.

Touching one of these tiles brings up a numeric keypad for direct entry.

SR865 DSP Lock-in Amplifier

Chapter 3 Operation 75

Numeric entry is straightforward. {Close} will return to the data screen. The buttons
{F1}, {F2}, {F3} and {F4} are frequency presets. Touching a preset will load the preset
value immediately. Touch and hold a preset button to memorize the current setting. Other
parameters may have slightly different entry screens.

Source [Ref/chop]

Press this key briefly to cycle through the reference sources. Press and hold to display the
SR540 Chopper setup screen (see Chop, below).

Internal

When the source is Internal, the SR865's synthesized internal oscillator is used as the
reference. The Ref In BNC is ignored in this case. The internal frequency is shown in the
info bar. Use the <Frequency> knob or the numeric keypad to adjust the frequency.

In this mode, the Sine Out is at the internal freque ncy .

External

When the source is External, the SR865 will phase lock to the external reference
provided at the Ref In BNC. The SR865 will lock to frequencies between 0.001 Hz and

2.5 MHz. The external frequency is shown in the info bar. The <Frequency> knob has no
effect on the external frequency.

In this mode, the Sine Out is at the external frequency.

Dual

When the source is Dual, the SR865 will detect signals at f
the external reference frequency and f

is the internal frequency. The internal frequency

int

is shown in the info bar. Use the <Frequency> knob or the numeric keypad to adjust the
internal frequency.

detect

= |f

int

− f

| where f

ext

ext

is

In this mode, the Sine Out is at the internal freque ncy .

To record and view the external frequency, use the [Config] key to assign f
the four data channels.

You can find more about dual reference mode in Appendix E (page 175).

SR865 DSP Lock-in Amplifier

to one of

ext

76 Operation Chapter 3

Chop

When the source is Chop, the SR865 will phase lock an external SR540 Optical Chopper
to the SR865’s internal reference frequency. This is achieved by connecting the f
Reference output from the SR540 to the Ref In of the SR865. But instead of using this
input as the external reference, the SR865 will servo the SR540 to lock the SR540 to the
SR865 internal frequency. This control is via a co nnec tion from the SR865 rear panel
Aux Out 4 BNC to the CONTROL VOLTAGE input of the SR540.

This essentially transforms the SR540 into a frequency and phase stabilized chopper with
the frequency accuracy and drift of the SR865. This is especially useful in experim ents
with multiple SR540 choppers. Each SR540 chopper (with its own SR865) can be
synchronized to a common 10 MHz clock to achieve stable frequency and phase
relationships with each other.

Lock-in detection is at the (measured) extern al frequency. The SR540 is driven to phase
lock with this int ern a l frequency, but is su b jec t to th e mechanical lim ita t ions of its motor
and slew rate. The internal frequency is shown in the info bar. Use the <Frequency> knob
or the numeric keypad to adjust the internal frequency. To record and view the external
frequency, use the [Config] key to assign f
comparing f

ext

and f

, the effective settling time of the SR540 to commanded frequency

int

changes can be observed.

to one of the four data channels. By

ext

In this mode, the Sine Out is at the external frequency.

Both the SR540 and the SR865 must be configured correctly in this mode. Press and hold
the [Ref/chop] key to display the Chopper configuration screen.

Touch the Chopper Blade Slot Count setting to specify whether the 6/5 or 30/25 slot
blade is mounted on the SR540 chopper head. Follow the directions to set the MAX
FREQ and REFERENCE MODE switches on the SR540.

Connect the SR540 f / f

output (right hand BNC) to the SR865 Ref In BNC.

DIFF

Connect the SR865 Aux Out 4 (rear panel BNC) to the SR540 CONTROL VOLTAGE
input BNC.

SR865 DSP Lock-in Amplifier

Chapter 3 Operation 77

Make sure that the SR865 External Trig is set to Pos TTL and the Input is 1 MΩ. Adjust
the SR865 internal frequency to a value that the SR540 can reach with the selected blade.

The Phase setting in this screen is the ‘blade’ phase. This is the phase of the blade
opening relative to the optical detector at the base of the chopper head. When a single
chopper is being used, this is not important since the demodulator reference phase of the
SR865 can be adjusted to maximize the signal. When using multiple choppers, this
‘blade’ phase can be used to align multiple chopped beams to arrive in phase together at
the experiment. This would otherwise be done by moving the position of the chopper or
beam.

The <Cursor> knob will also adjust the phase value in this screen.

External [Input]

This key sets the termination of the Ref Input BNC to either 50 Ω or 1 MΩ. Be careful
that the termination does excessively attenuate the signal or cause ringing on its edges.

External [Trig]

This key selects the external reference input trigger mode.

TTL

When either Pos TTL or Neg TTL is selected, the SR865 locks to the selected edge of a
TTL square wave or pulse train. For reliable operation, the TTL signal should exceed
+1.5 V when high and be less than +0.5 V when low. This trigger mode is dc coupled.
This input mode should be used whenever possible since it is less noise prone than the
sine wave discriminator.

For very low frequencies (<1 Hz), a TTL reference MUST be used.

Sine

Sine input trigger locks the SR865 to the rising zero crossings of an analog signal at the
Ref In BNC. This signal should be a clean sine wave at least 200 mVpp in amplitude. In
this input mode, the Ref In is ac coupled (above 1 Hz).

Sine reference mode cannot be used at frequencies far below 1 Hz. At very low
frequencies, the TTL input modes must be used.

Unlock

In External or Dual reference source, the Unlock indicator turns on if the SR865 cannot
lock to the external reference. This can be because the external reference am plitud e is too
low or the frequency is out of range.

In Chop reference,
because the switches on the SR540 are incorrectly configured or the blade is not turning
freely.

Unlock indicates that the SR540 chopper is unlocked. This can be

SR865 DSP Lock-in Amplifier

78 Operation Chapter 3

Timebase

The SR865 has a 10 MHz TIMEBASE input and output on the rear panel. Apply a
10 MHz sine signal (1 Vrms) to the TIMEBASE input to lock the SR865’s timebase to an
external 10 MHz timebase. The internal reference frequency of the SR865 is derived
from this timebase. When multiple SR865’s are locked to the same 10 MHz timebase,
then harmonically related internal frequencies on different units will stay in phase
indefinitely.

The

Ext 10 MHz LED is on when the unit is locked to an external timebas e.

The SR865 can also output its own 10 MHz timebase to another unit.

Note that external function generato rs, ev en DD S locked to the sam e 10 MHz timebase,
will not stay in perfect phase with the SR865. This is because the resolution of the
frequency tuning word in the external synthesizer differs from the SR865.

<Phase>

This knob may be used to adjust the phase. The phase shift ranges from −180° to +180°
with 0.001° resolution. Press and hold this knob to set the phase to zero.

The phase value is shown in the info bar at the top of the screen. Touch this {Phase} tile
to show the numeric entry screen.

When using an external reference, the refere nce phas e shif t is the phase betw een t he
external reference and the digital sine wave which is multiplying the signal in the PSD.
This is also the phase between the sine output and the digital sine wave used by the PSD
in either internal or external reference mode. Changing this phase shift only shifts internal
sine waves. The effect of this phase shift can only be seen at the lock-in outputs X, Y and
θ. R is phase independent.

Auto Phase

Touching {Auto} in the numeric entry screen or pressing the [Auto Phase] key will adjust
the reference phase shift so that the measured signal phase is 0°. This is done by
subtracting the present measured value of θ from the reference phase shift. It will take
several time constants for the outputs to reach their new values. Auto Phase may not
result in a zero phase if the measurement is noisy or changing.

Phase [+90°] and [−90°]

The [+90°] and [−90°] keys add or subtract 90.000° from the reference phase shift.

Touching {+90°} and {−90°} in the phase numeric entry screen will do the same.

<Frequency>

If the reference source is Internal, Dual or Chop, this knob adjusts the internal reference
frequency.

The internal frequency is shown in the info bar at the top of the screen. Touch the {F
tile to show the numeric entry screen.

SR865 DSP Lock-in Amplifier

}

int

Chapter 3 Operation 79

The internal frequency has 6 digits or 0.1 mHz resolution, whichever is larger. The
frequency can range from 1.0 mHz to 2.50 MHz. The upper limit is decreased if the
harmonic number is greater than 1. In this case, the upper limit is 2.5 MHz/N where N is
the harmonic number.

Press and hold this knob to set the harmonic detect to 1 (fundamental).

Frequency [Harm Up] and [Har m Down]

Pressing these keys increments and decrements the harmonic detect number from 1 to 99.

The harmonic number is shown in the info bar at the top of the screen. Touch this
{Harm} tile to show the numeric entry screen.

The SR865 can detect signals at harmonics of the reference frequency. The SR865
multiplies the input signal with digital sine waves at N×f
will be detected. Signals at the original reference frequency are not detected and are
attenuated as if they were noise. Always check the harmonic detect number before
making any measurements.

. Only signals at this harmonic

ref

The Sine Out is not affected by the harmonic detection. Its frequency is always the
fundamental reference freq uen cy .

Internal and Chop Reference

When the reference mode is Internal or Chop, these keys change the internal harmonic
number N

int

2.5 MHz/N

External Reference

When the reference mode is External, these keys change the external harmonic number
N

and detection is at N

ext

Dual Reference

When the reference mode is Dual, detection is at f
harmonic numeric entry screen to adjust either harmonic number. The limits described
above apply. The Sine Out will still be at f

<Amplitude>

This knob adjusts the sine out amplitude.

The amplitude is shown in the info bar at the top of the screen. Touch the {Ampl} tile to
show the numeric entry screen.

and detection is at N

.

int

ext×fext

and the internal reference frequency is lim ite d to

int×fint

. The SR865 will always track the external reference.

= | N

dual

.

int

int×fint

− N

ext×fext

|. Use the

The amplitude has 3 digits or 1 nV resolution, whichever is larger. The amplitude can
range from 1 nVrms to 2.00 Vrms.

Press and hold this knob to set the amplitude to zero.

The Sine Out amplitude is specified for differential output (Sine+) − (Sine−). Each output

BNC has the specified rms amplitude through a 50Ω output impedance. If an output is

SR865 DSP Lock-in Amplifier

80 Operation Chapter 3

terminated in a high impedance, the amplitude will be the specified value. If an output is

terminated in 50Ω, the amplitude will be half the specified value. The outputs are 180°

out of phase. If both outputs are used differentially, the amplitude will be twice the output
of a single BNC.

In gen eral, if single output is used as the A input, the measured signal will be the
specified amplitude (a single output terminated in a high impedance).

Using differential signals can be helpful when the sine amplitude is very small.

When the reference source is Internal or Dual, this is the excitation source provided by
the SR865. When an external reference is used (or in Chop mode), this sine output
provides a sine wave phase locked to the external reference.

The rear panel BlazeX BNC output can be configured in the System menu (hold
[Calc/system]) to provide a square wave sync at the reference frequency. This square
wave is generated by discriminating the zero crossings of the sine output. This signal can
provide a trigger or sync signal to the experiment when the internal reference source is
used. This signal is also available when the reference is externally provided. In this case,
the sync output is phase locked to the external reference.

<DC Level>

This knob adjusts the sine out dc level.

The dc level is shown in the info bar at the top of the screen. Touch the {DC} tile to show
the numeric entry screen.

The dc level has 3 digits or 0.1 mV resolution, whichever is larger. The dc lev el ra ng es
from −5.00 V to +5.00 V.

Press and hold this knob to set the dc level to zero.

The dc level adds an offset to the sine output. Each output BNC has the specified dc level
through a 50Ω output impedance. If an output is terminated in a high impedance, the dc
level will be the specified value. If an output is terminated in 50Ω, the dc level will be
half the specified value.

The dc level is added to the outputs in Difference mode (opposite sign dc level on each
BNC) or in Common mode (same sign dc level on both BNC’s). Choose the mode with
the [Mode] key.

DC Level [Mode]

This key selects the dc level mode. The dc level is added to the sine outputs in Difference
mode (opposite sign dc level on each BNC) or in Common mode (same sign dc level on
both BNC’s).

SR865 DSP Lock-in Amplifier

Chapter 3 Operation 81

Display

[Screen Layout]

Press [Screen Layout] to cycle through the different screen layouts.

Trend Graph Full Screen Strip Chart

Half Screen Strip Chart Full Screen FFT

Half Screen FFT Big Numbers

[Screen Shot]

Pressing [Screen Shot] saves a screen shot to the USB memory stick as a .BMP file. Set
the Print Mode in the system menu. Screen is an exact screen shot, Print replaces the

SR865 DSP Lock-in Amplifier

82 Operation Chapter 3

black background with white and Monochrome is black and white. Screen shots also
contain the time/date and summary of input parameters.

The

Busy LED indicates that the USB stick is busy and should not be removed.

[Blank]

Press the [Blank] key to operate with the front panel display and LEDs off. The SR865 is
still operating, the outputs are active, data collection continues and the unit responds to
interface commands. To change a setting, press [Blank] to return to normal operation,
change the desired parameter, then press [Blank] again.

The

Blank LED is lit while the rest of the display is off.

[Config]

The SR865 displays up to 4 channels at a time, in green, blue, yellow and orange. Each
channel is assigned a parameter using the [Config] key. Parameters are chosen from X,

Y, R, θ (detected), F

or Output, Xnoise, or Ynoise .

, F

, Reference Phase, Sine Amplitude, DC Level, any Aux Input

int

ext

Highlight one of the channel’s Display tile, then select a parameter from below to assig n
it to the channel.

Displayed parameters can be re-assigned at any time. Data is being stored for all
parameters all of the time.

SR865 DSP Lock-in Amplifier