SRS Labs SR850 User Manual

MODEL SR850

DSP Lock-In Amplifier

Stanford Research Systems

1290-D Reamwood Avenue

Sunnyvale, California 94089

Phone: (408) 744-9040 • Fax: (408) 744-9049

email: info@thinkSRS.com • www.thinkSRS.com

Revision 1.8 (02/2007)

TABLE OF CONTENTS

GENERAL INFORMATION

Safety and Preparation for Use 1-3 Specifications 1-5 Abridged Command List 1-7

GETTING STARTED

Your First Measurements 2-1 The Basic Lock-in 2-3 Displays and Traces 2-7 Outputs, Offsets and Expands 2-13 Scans and Sweeps 2-17 Using the Disk Drive 2-23 Aux Outputs and Inputs 2-31 Trace Math 2-35

SR850 BASICS

What is a Lock-in Amplifier? 3-1 What Does a Lock-in Measure? 3-3 The SR850 Functional Diagram 3-5 Reference Channel 3-7 Phase Sensitive Detectors 3-9 Time Constants and DC Gain 3-11 DC Outputs and Scaling 3-13 Dynamic Reserve 3-17 Signal Input Amplifier and Filters 3-19 Input Connections 3-21 Intrinsic (Random) Noise Sources 3-23 External Noise Sources 3-25 Noise Measurements 3-27

OPERATION

FRONT PANEL 4-1 Power On/Off and Power On Tests 4-1 Video Display 4-1 Soft Keys 4-2 Keypad 4-2 Spin Knob 4-2 Disk Drive 4-2 Front Panel BNC Connectors 4-2

SCREEN DISPLAY 4-5 Default Display 4-5 Data Traces 4-6 Single/Dual Trace Displays 4-7 Bar Graphs 4-9 Polar Graphs 4-10 Strip Charts 4-11 Trace Scans, Sweeps and Aliasing 4-13 Settings and Input/Output Monitor 4-15 Menu Display 4-15 Status Indicators 4-16

KEYPAD 4-19 Normal and Alternate Keys 4-19 Menu Keys 4-19

Additional Menus 4-20 Entry Keys 4-20 START/CONT and PAUSE/RESET 4-20 CURSOR 4-21 ACTIVE DISPLAY 4-21 MARK 4-21 CURSOR MAX/MIN 4-22 AUTO RESERVE 4-22 AUTO GAIN 4-22 AUTO PHASE 4-22 AUTO SETUP 4-22 AUTOSCALE 4-22 PRINT to a PRINTER 4-23 PRINT to a FILE 4-23 HELP 4-23 LOCAL 4-23

REAR PANEL 4-25 Power Entry Module 4-25 IEEE-488 Connector 4-25 RS232 Connector 4-25 Parallel Printer Connector 4-25 PC Keyboard Connector 4-25 Rear Panel BNC Connectors 4-26 Aux Inputs (A/D Inputs) 4-26 Aux Outputs (D/A Outputs) 4-26 X and Y Outputs 4-26 Signal Monitor Output 4-26 Trigger Input 4-27 TTL Sync Output 4-27 Preamp Connector 4-27

USING SRS PREAMPS 4-27

MENUS

Menu Guide 5-1 Default Settings 5-2 Reference and Phase Menu 5-3 Input and Filters Menu 5-7 Gain and Time Constant Menu 5-9 Output and Offset Menu 5-15 Trace and Scan Menu 5-17 Display and Scale Menu 5-21 Aux Outputs Menu 5-25 Cursor Setup Menu 5-29 Edit Mark Menu 5-31 Math Menu 5-33 Disk Menu 5-41 System Setup Menu 5-49

PROGRAMMING

GPIB Communications 6-1 RS232 Communications 6-1 Status Indicators and Queues 6-1 Command Syntax 6-1

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Table of Contents

Interface Ready and Status 6-2 GET (Group Execute Trigger) 6-2

DETAILED COMMAND LIST 6-3 Reference and Phase 6-4 Input and Filter 6-6 Gain and Time Constant 6-7 Output and Offset 6-9 Trace and Scan 6-10 Display and Scale 6-11 Cursor 6-13 Mark 6-14 Aux Input and Output 6-15 Math 6-16 Store and Recall 6-18 Setup 6-19 Print and Plot 6-21 Front Panel and Auto Functions 6-22 Data Transfer 6-23 Interface 6-28 Status Reporting 6-29

STATUS BYTE DEFINITIONS 6-30 Serial Poll Status Byte 6-30 Service Requests 6-31 Standard Event Status Byte 6-31 LIA Status Byte 6-32 Error Status Byte 6-32

PROGRAM EXAMPLES Microsoft C, Nationall Instr GPIB 6-33 QUICKBASIC, Nationall Instr GPIB 6-39

TESTING

Introduction 7-1 Preset 7-1 Serial Number 7-1 Firmware Revision 7-1 General Installation 7-2 Necessary Equipment 7-3 If A Test Fails 7-3

PERFORMANCE TESTS Self Tests 7-5 DC Offset 7-7 Common Mode Rejection 7-9 Amplitude Accuracy and Flatness 7-11 Amplitude Linearity 7-13 Frequency Accuracy 7-15 Phase Accuracy 7-17 Sine Output Amplitude 7-19 DC Outputs and Inputs 7-21 Input Noise 7-23

PERFORMANCE TEST RECORD 7-25

SERVICE Circuit Boards 8-1

Adjusting DC Offset and CMRR 8-3 Adjusting Notch Filters 8-6

CIRCUITRY

Circuit Boards 9-1 Video Driver and CRT 9-1 CPU Board 9-3 Power Supply Board 9-5 DSP Logic Board 9-7 Analog Input Board 9-9

PARTS LISTS Power Supply Board 9-11 DSP Logic Board 9-13 Analog Input Board 9-20 CPU Board 9-27 Chassis Assembly 9-32 Miscellaneous 9-35

SCHEMATIC DIAGRAMS CPU Board Power Supply Board DSP Logic Board Analog Input Board

1-2

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.

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.

LINE VOLTAGE SELECTION

The SR850 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 to a power source, 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 visible.

Conversion to other AC input voltages requires a change in the fuse holder voltage card position and fuse value. Disconnect the power cord, open the fuse holder cover door and rotate the fuse-pull lever to remove the fuse. Remove the small printed circuit board and select the operating voltage by orienting the printed circuit board so that the desired voltage is visible when pushed firmly into its slot. Rotate the fuse-pull lever back into its normal position and insert the correct fuse into the fuse holder.

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.

LINE CORD

The SR850 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.

SERVICE

Do not attempt to service or adjust 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.

FURNISHED ACCESSORIES

- Power Cord

- Operating Manual

ENVIRONMENTAL CONDITIONS

OPERATING Temerpature: +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

WARNING REGARDING USE WITH PHOTOMULTIPLIERS 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 SR850 the stored charge may damage the front-end op amps. To avoid this problem, always discharge the cable and connect the PMT output to the SR850 input before turning the PMT on.

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SR850 DSP LOCK-IN AMPLIFIER

SPECIFICATIONS

SIGNAL CHANNEL

Voltage Inputs Single-ended (A) or differential (A-B). Current Input 106 or 108 Volts/Amp. Full Scale Sensitivity 2 nV to 1 V in a 1-2-5-10 sequence (expand off). Input Impedance Voltage: 10 MΩ+25 pF, AC or DC coupled.

Current: 1 kΩ to virtual ground. Gain Accuracy ±1% from 20°C to 30°C (notch filters off), ±0.2% typical. Input Noise 6 nV/√Hz at 1 kHz (typical). Signal Filters 60 (50) Hz and 120(100) Hz notch filters (Q=4). CMRR 100 dB at 10 kHz (DC Coupled), decreasing by 6 db/octave above 10 kHz Dynamic Reserve Greater than 100 dB (with no signal filters). Harmonic Distortion <-90 dB to 10 kHz, <-80 dB to 100 kHz.

REFERENCE CHANNEL

Frequency Range 1 mHz to 102 kHz Reference Input TTL (rising or falling edge) or Sine.

Sine input is1 MΩ, AC coupled (>1 Hz). 400 mV pk-pk minimum signal. Phase Resolution 0.001° Absolute Phase Error <1° Relative Phase Error <0.001° Orthogonality 90° ± 0.001° Phase Noise External synthesized reference: 0.005° rms at 1 kHz, 100 ms, 12 dB/oct.

Internal reference: crystal synthesized, <0.0001° rms at 1 kHz. Phase Drift <0.01°/°C below 10 kHz

<0.1°/°C to 100 kHz Harmonic Detect Detect at Nxf where N<32767 and Nxf<102 kHz. Acquisition Time (2 cycles + 5 ms) or 40 ms, whichever is greater.

DEMODULATOR

Zero Stability Digital displays have no zero drift on all dynamic reserves.

Analog outputs: <5 ppm/°C for all dynamic reserves. Time Constants 10 µs to 30 s (reference > 200 Hz). 6, 12, 18, 24 dB/oct rolloff.

up to 30000 s (reference < 200 Hz). 6, 12, 18, 24 dB/oct rolloff.

Synchronous filtering available below 200 Hz. Harmonic Rejection -90 dB

INTERNAL OSCILLATOR

Frequency 1 mHz to 102 kHz. Frequency Accuracy 25 ppm + 30 µHz Frequency Resolution 5 digits or 0.1 mHz, whichever is greater. Frequency Sweeps Linear and Log. Distortion f<10 kHz, below -80 dBc. f>10 kHz, below -70 dBc.1 Vrms amplitude. Output Impedance 50 Ω Amplitude 4 mVrms to 5 Vrms (into a high impedance load) with 2 mV resolution.

(2 mVrms to 2.5 Vrms into 50Ω load). Amplitude Accuracy 1% Amplitude Stability 50 ppm/°C Outputs Sine output on front panel. TTL sync output on rear panel.

When using an external reference, both outputs are phase locked to the

external reference.

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SR850 DSP Lock-In Amplifier

INPUTS AND OUTPUTS

Channel 1 Output X, R, θ, or Trace 1-4. Traces are defined as A•B/C or A•B/C2 where A, B,

and C are selected from the quantities Unity, X, Y, R, θ, Xnoise, Ynoise, Rnoise, Aux Inputs 1 through 4, or Frequency. Output Voltage: ±10 V full scale. 10 mA max output current.

Channel 2 Output Y, R, θ, or Trace 1-4. Traces are defined as A•B/C or A•B/C2 where A, B,

and C are selected from the quantities Unity, X, Y, R, θ, Xnoise, Ynoise, Rnoise, Aux Inputs 1 through 4, or Frequency. Output Voltage: ±10 V full scale. 10 mA max output current.

X and Y Outputs Rear panel outputs of cosine (X) and sine (Y) components.

Output Voltage: ±10 V. 10 mA max output current.

Aux. Outputs 4 BNC Digital to Analog outputs.

±10 V full scale, 1 mV resolution. May be set to a fixed voltage or swept in amplitude (linear or log). 10 mA max output current.

Aux. Inputs 4 BNC Analog to Digital inputs.

Differential inputs with1 MΩ input impedance on both shield and center

conductor. ±10 V full scale, 1 mV resolution. Trigger Input TTL trigger input triggers each data sample and/or start of scan. Monitor Output Analog output of signal amplifiers (before the demodulator).

DISPLAYS

Screen Format Single or dual display. Displayed Quantities Each display may show one of the traces.

Traces are defined as A•B/C or A•B/C2 where A, B and C are selected from

the quantities Unity, X, Y, R, θ, Xnoise, Ynoise, Rnoise, Aux Inputs 1

through 4, or Frequency. Display Types Large numeric readout with bar graph, polar graph, and strip chart. Chart Data Buffer 64k data points may be stored and displayed on strip charts. The buffer can

be configured as a single trace with 64k points, 2 traces with 32k points

each, or 4 traces with16k points each. The internal data sample rate ranges

from 512 Hz down to 1 point every 16 seconds. Samples can also be

triggered.

ANALYSIS FUNCTIONS

Smoothing 5 - 25 point Savitsky-Golay smoothing of trace regions. Curve Fits Line, Exponential, and Gaussian fits of trace regions. Calculator Arithmetic, trigonometric, and logarithmic calculations on trace regions. Statistics Mean and standard deviation of trace regions.

GENERAL

Monitor Monochrome CRT. 640H by 480V resolution.

Adjustable brightness and screen position. Interfaces IEEE-488, RS232 and Printer interfaces standard.

All instrument functions can be controlled through the IEEE-488 and RS232

interfaces. A PC keyboard input is provided for additional flexibility. Preamp Power Power connector for SR550 and SR552 preamplifiers. Hardcopy Screen dumps to dot matrix and HP LaserJet compatible printers. Data

plots to HP-GL compatible plotters (via RS232 or IEEE-488). Screens can

also be saved to disk as PCX image files. Disk 3.5 inch DOS compatible format, 720 kbyte capacity.

Storage of data and setups. Power 60 Watts, 100/120/220/240 VAC, 50/60 Hz. Dimensions 17"W x 6.25"H x 19.5"D Weight 40 lbs. Warranty One year parts and labor on materials and workmanship.

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SR850 DSP Lock-In Amplifier

COMMAND LIST

VARIABLES i,j,k,l,m Integers

f Frequency (real) x,y,z Real Numbers s String

REFERENCE and PHASE page description PHAS (?) {x} 6-4 Set (Query) the Phase Shift to x degrees. FMOD (?) {i} 6-4 Set (Query) the Reference Source to Internal (0), Sweep (1) , or External (2). FREQ (?) {f} 6-4 Set (Query) the Reference Frequency to f Hz.Set only in Internal reference mode. SWPT (?) {i} 6-4 Set (Query) the Internal Sweep Type to Linear (0) or logarithmic (1). SLLM (?) {f} 6-4 Set (Query) the Start Frequency to f Hz.Set only in Internal Sweep mode. SULM (?) {f} 6-4 Set (Query) the Stop Frequency to f Hz.Set only in Internal Sweep mode. RSLP (?) {i} 6-4 Set (Query) the External Reference Slope to Sine(0), TTL Rising (1), or TTL Falling (2).

HARM (?) {i} 6-5 Set (Query) the Detection Harmonic to 1 ≤ i ≤ 32767 and i•f ≤ 102 kHz. SLVL (?) {x} 6-5 Set (Query) the Sine Output Amplitude to x Vrms. 0.004 ≤ x ≤5.000.

INPUT and FILTER page description ISRC (?) {i} 6-6 Set (Query) the Input Configuration to A (0), A-B (1) , or I (2).

IGAN (?) {i} 6-6 Set (Query) the Current Conversion Gain to 1 MΩ (0) or 100 MΩ (1).

IGND (?) {i} 6-6 Set (Query) the Input Shield Groungind to Float (0) or Ground (1). ICPL (?) {i} 6-6 Set (Query) the Input Coupling to AC (0) or DC (1). ILIN (?) {i} 6-6 Set (Query) the Line Notch Filters to Out (0), Line In (1) , 2xLine In (2), or Both In (3).

GAIN and TIME CONSTANT page description SENS (?) {i} 6-7 Set (Query) the Sensitivity to 2 nV (0) through 1 V (26) rms full scale. RMOD (?) {i} 6-7 Set (Query) the Dynamic Reserve Mode to Max (0), Manual (1), or Min (2). RSRV (?) {i} 6-7 Set (Query) the Dynamic Reserve to i th reserve. Set will switch to Manual Reserve Mode. OFLT (?) {i} 6-7 Set (Query) the Time Constant to 10 µs (0) through 30 ks (19). OFSL (?) {i} 6-8 Set (Query) the Low Pass Filter Slope to 6 (0), 12 (1), 18 (2) or 24 (3) dB/oct. SYNC (?) {i} 6-8 Set (Query) the Synchronous Filter to Off (0) or On below 200 Hz (1).

OUTPUT and OFFSET page description

FOUT (?) i {, j} 6-9 Set (Query) the CH1 (i=1) or CH2 (2) Output Source to XY,R,θ,Trace 1, 2, 3, 4 (j=0...6). OEXP (?) i {, x, j} 6-9 Set (Query) the X, Y, R (i=1,2,3) Offset to x percent and Expand to j. -105.00 ≤ x ≤ 105.00 and

1 ≤ j ≤ 256.

AOFF i 6-9 Auto Offset X, Y, R (i=1,2,3).

TRACE and SCAN page description TRCD (?) i {, j, k, l, m} 6-10 Set (Query) the Definition of Trace i (1-4) to j•k/l and Store (m=1) or Not Store (0). j, k, l select

1, X, Y, R, q, Xn, Yn, Rn, Aux 1, Aux 2, Aux 3, Aux4, or F (j,k,l = 0...12). l=13-24 selects X

through F2. SRAT (?) {i} 6-10 Set (Query) the Sample Rate to 62.5 mHz (0) through 512 Hz (13) or Trigger (14). SLEN (?) {x} 6-10 Set (Query) the Scan Length to x seconds. SEND (?) {i} 6-10 Set (Query) the Scan Mode to 1 Shot (0) or Loop (1). TRIG 6-10 Software trigger command. Same as trigger input.

DISPLAY and SCALE page description ASCL 6-11 Auto Scale the active display. ADSP (?) {i} 6-11 Set (Query) the active display to Full (0), Top (1) or Bottom (2). Full screen is always active. SMOD (?) {i} 6-11 Set (Query) the Screen Format to Single (0) or Up/Down dual (1) display mode. MNTR (?) {i} 6-11 Set (Query) the Monitor Display to settings (0) or Input/Output (1). DTYP (?) i {, j} 6-11 Set (Query) theFull (i=0), Top (i=1) or Bottom (i=2) Display Type to Polar (j=0), Blank (j=1),

Bar (j=2) or Chart (j=3).

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SR850 DSP Lock-In Amplifier

DTRC (?) i {, j} 6-11 Set (Query) theFull (i=0), Top (i=1) or Bottom (i=2) Display Trace to trace j (1,2,3,4). DSCL (?) {x} 6-11 Set (Query) theFull (i=0), Top (i=1) or Bottom (i=2) Display Range to x. DOFF (?) {x} 6-11 Set (Query) theFull (i=0), Top (i=1) or Bottom (i=2) Display Center value to x. DHZS (?) {i} 6-12 Set (Query) theFull (i=0), Top (i=1) or Bottom (i=2) Display Horizontal Scale to 2 ms (0) through

200 ks (32) per div.

RBIN? i 6-12 Query the bin number at the right edge of the Full (i=0), Top (i=1) or Bottom (i=2) chart display.

CURSOR page description CSEK (?) {i} 6-13 Set (Query) the active display Cursor Seek mode to Max (0), Min (1) or Mean (2). CWID (?) {i} 6-13 Set (Query) the active display Cursor Width to Off (0), Narrow (1), Wide (2) or Spot (3). CDIV (?) {i} 6-13 Set (Query) the active display Chart Divisions to 8 (0), 10 (1) or None (2). CLNK (?) {i} 6-13 Set (Query) the Cursor Control Mode to Linked (0) or Separate (1). CDSP (?) {i} 6-13 Set (Query) the active display Cursor Readout to Delay (0), Bin (1), Fsweep (2) or Time (3). CMAX 6-13 Move active chart cursor to max or min. Same as pressing [CURSOR MAX/MIN] key. CURS? i 6-13 Query the cursor horz,vert position of Full (0), Top (1) or Bottom (2) chart display. CBIN (?) {i} 6-13 Set (Query) the center of the cursor region in the active chart display. i is the bin number.

MARK page description MARK 6-14 Places a mark in the data buffer. Same as pressing [MARK] key. MDEL 6-14 Delete the nearest mark to the left of the cursor. Same as pressing <Marker Delete> softkey. CNXT 6-14 Move active chart cursor to next mark to the right. CPRV 6-14 Move active chart cursor to next mark to the left. MACT? 6-14 Query the number of active marks. Also returns the active mark numbers. MBIN? i 6-14 Query the bin number of mark #i. MTXT (?) i {,s} 6-14 Set (Query) the label text for mark #i.

AUX INPUT/OUTPUT page description OAUX ? i 6-15 Query the value of Aux Input i (1,2,3,4). AUXM(?) i{, j} 6-15 Set (Query) the Output Mode of Aux Output i (1,2,3,4). j selects Fixed (0), Log (1) or Linear (2).

AUXV (?) i {, x} 6-15 Set (Query) voltage of Aux Output i (1,2,3,4) to x Volts. -10.500 ≤ x ≤ 10.500. Fixed Output

Mode only.

SAUX (?) i {, x, y, z} 6-15 Set (Query) the Aux Output i (1,2,3,4) Sweep Limits to Start (x), Stop (y) and Offset (z) voltag-

es. 0.001 ≤ x,y ≤ 21.000 and -10.500 ≤ z ≤ 10.500.

TSTR (?) {i} 6-15 Set (Query) the Trigger Starts Scan? mode to No (0) or Yes (1).

MATH page description SMTH i 6-16 Smooth the data within the active chart using 5 (0), 11 (1), 17 (2), 21 (3), 25 (4) point width.

COPR (?) {i} 6-16 Set (Query) the Calculator Operation to +, -, x, /, sin, cos, tan, √x, x

CALC 6-17 Do the Calculation selected by COPR with the argument set by CTRC or CARG. CAGT (?) {i} 6-17 Set (Query) the Calculation Argument Type to Trace (0) or Constant (1). CTRC (?) {i} 6-17 Set (Query) the Trace Argument to Trace i (1,2,3,4). CARG (?) {x} 6-17 Set (Query) the Constant Argument value to x. FTYP (?) {i} 6-17 Set (Query) the Fit Type to Linear (0), Exponential (1) or Gaussian (2).

FITT i, j 6-17 Fit the data within the chart region between i% and j% from the left edge. 0 ≤ i,j ≤ 100.

PARS ? i 6-17 Query the fit parameters a (0), b (1), c (2) or t0 (3). STAT i, j 6-17 Statistically analyze the data within the chart region between i% and j% from the left edge.

0 ≤ i,j ≤ 100.

SPAR ? i 6-17 Query the Statistical results mean (0), standard dev (1), total (2) or delta time (3).

STORE AND RECALL FILE page description FNAM (?) {s} 6-18 Set (Query) the current File Name to string s. SDAT 6-18 Save the Active Display's Trace Data to the file specified by FNAM. SASC 6-18 Save the Active Display's Trace Data in ASCII format to the file specified by FNAM. SSET 6-18 Save the Settings to the file specified by FNAM.

, log, 10x (i=0...10).

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SR850 DSP Lock-In Amplifier

RDAT 6-18 Recall the Trace Data from the file specified by FNAM to the active display's trace buffer. RSET 6-18 Recall the Settings from the file specified by FNAM.

SETUP page description OUTX (?) {i} 6-19 Set (Query) the Output Interface to RS232 (0) or GPIB (1). OVRM (?) {i} 6-19 Set (Query) the GPIB Overide Remote state to Off (0) or On (1). KCLK (?) {i} 6-19 Set (Query) the Key Click to Off (0) or On (1). ALRM (?) {i} 6-19 Set (Query) the Alarms to Off (0) or On (1).

THRS (?) {i} 6-19 Set (Query) the Hours to 0≤ i ≤ 23. TMIN (?) {i} 6-19 Set (Query) the Minutes to 0 ≤ i ≤ 59. TSEC (?) {i} 6-19 Set (Query) the Seconds to 0 ≤ i ≤ 59. DMTH (?) {i} 6-19 Set (Query) the Month to 1 ≤ 1 ≤ 12. DDAY (?) {i} 6-19 Set (Query) the Day to 1 ≤ 1 ≤ 31. DYRS (?) {i} 6-19 Set (Query) the Year to 0 ≤ 1 ≤ 99.

PLTM (?) {i} 6-19 Set (Query) the Plotter Mode to RS232 (0) or GPIB (1). PLTB (?) {i} 6-19 Set (Query) the Plotter Baud Rate to 300 (0), 1200 (1), 2400 (2), 4800 (3), 9600 (4).

PLTA (?) {i} 6-19 Set (Query) the Plotter GPIB Address to 0 ≤ i ≤ 30.

PLTS (?) {i} 6-19 Set (Query) the Plot Speed to Fast (0) or Slow (1).

PNTR (?) {i} 6-19 Set (Query) the Trace Pen Number to 1 ≤ i ≤ 6. PNGD (?) {i} 6-20 Set (Query) the Grid Pen Number to 1 ≤ i ≤ 6. PNAL (?) {i} 6-20 Set (Query) the Alphanumeric Pen Number to 1 ≤ i ≤ 6. PNCR (?) {i} 6-20 Set (Query) the Cursor Pen Number to 1 ≤ i ≤ 6.

PRNT (?) {i} 6-20 Set (Query) the Printer Type to Epson (0), HP (1) or File (2).

PRINT AND PLOT page description PRSC 6-21 Print the screen. Same as the [PRINT] key. PALL 6-21 Plot the display(s). PTRC 6-21 Plot the trace(s) only. PCUR 6-21 Plot the cursor(s) only.

FRONT PANEL CONTROLS AUTO FUNCTIONS page description

STRT 6-22 Start or continue a scan. Same as [START/CONT] key. PAUS 6-22 Pause a scan. Does not reset a paused or done scan. REST 6-22 Reset the scan. All stored data is lost. ASCL 6-11 Auto Scale the active display. ATRC (?) {i} 6-22 Set (Query) the active display to Top (0) or Bottom (1). Full screen is always active. AGAN 6-22 Auto Gain function. Same as pressing the [AUTO GAIN] key. ARSV 6-22 Auto Reserve function. Same as pressing the [AUTO RESERVE] key. APHS 6-22 Auto Phase function. Same as pressing the [AUTO PHASE] key. AOFF i 6-22 Auto Offset X,Y or R (i=1,2,3). CMAX 6-22 Move Cursor to Max or Min. Same as pressing the [CURSOR MAX/MIN] key.

DATA TRANSFER page description

OUTP? i 6-23 Query the value of X (1), Y (2), R (3) or θ (4). Returns ASCII floating point value.

OUTR? i 6-23 Query the value of Trace i (1,2,3,4). Returns ASCII floating point value. OAUX? i 6-23 Query the value of Aux Input i (1,2,3,4). Returns ASCII floating point value. SPTS? i 6-23 Query the number of points stored in Trace i (1,2,3,4).

TRCA? i,j,k 6-23 Read k≥1 points starting at bin j≥0 from trace i (1,2,3,4) in ASCII floating point. TRCB? i,j,k 6-23 Read k≥1 points starting at bin j≥0 from trace i (1,2,3,4) in IEEE binary floating point. TRCL? i,j,k 6-24 Read k≥1 points starting at bin j≥0 from trace i (1,2,3,4) in non-normalized binary floating point.

FAST (?) {i} 6-25 Set (Query) Fast Data Transfer Mode On (1 or 2) or Off (0).On will transfer binary X and Y eve-

ry sample during a scan over the GPIB interface. STRD 6-25 Start a scan after 0.5sec delay. Use with Fast Data Transfer Mode.

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SR850 DSP Lock-In Amplifier

INTERFACE page description *RST 6-26 Reset the unit to its default configurations. *IDN? 6-26 Read the SR850 device identification string. LOCL(?) {i} 6-26 Set (Query) the Local/Remote state to LOCAL (0), REMOTE (1), or LOCAL LOCKOUT (2). OVRM (?) {i} 6-26 Set (Query) the GPIB Overide Remote state to Off (0) or On (1). TRIG 6-26 Software trigger command. Same as trigger input.

STATUS page description *CLS 6-27 Clear all status bytes. *ESE (?) {i} {,j} 6-27 Set (Query) the Standard Event Status Byte Enable Register to the decimal value i (0-255).

*ESE i,j sets bit i (0-7) to j (0 or 1). *ESE? queries the byte. *ESE?i queries only bit i. *ESR? {i} 6-27 Query the Standard Event Status Byte. If i is included, only bit i is queried. *SRE (?) {i} {,j} 6-27 Set (Query) the Serial Poll Enable Register to the decimal value i (0-255). *SRE i,j sets bit i (0-

7) to j (0 or 1). *SRE? queries the byte, *SRE?i queries only bit i. *STB? {i} 6-27 Query the Serial Poll Status Byte. If i is included, only bit i is queried. *PSC (?) {i} 6-27 Set (Query) the Power On Status Clear bit to Set (1) or Clear (0). ERRE (?) {i} {,j} 6-27 Set (Query) the Error Status Enable Register to the decimal value i (0-255). ERRE i,j sets bit i

(0-7) to j (0 or 1). ERRE? queries the byte, ERRE?i queries only bit i. ERRS? {i} 6-27 Query the Error Status Byte. If i is included, only bit i is queried. LIAE (?) {i} {,j} 6-27 Set (Query) the LIA Status Enable Register to the decimal value i (0-255). LIAE i,j sets

bit i (0-7) to j (0 or 1). LIAE? queries the byte, LIAE?i queries only bit i. LIAS? {i} 6-27 Query the LIA Status Byte. If i is included, only bit i is queried.

STATUS BYTE DEFINITIONS

SERIAL POLL STATUS BYTE (6-28)

bit name usage 0 SCN No data is being acquired 1 IFC No command execution in progress 2 ERR Unmasked bit in error status byte set 3 LIA Unmasked bit in LIA status byte set 4 MAV The interface output buffer is non-empty 5 ESB Unmasked bit in standard status byte set 6 SRQ SRQ (service request) has occurred 7 Unused

STANDARD EVENT STATUS BYTE (6-29)

bit name usage 0 INP Set on input queue overflow 1 Unused 2 QRY Set on output queue overflow 3 Unused 4 EXE Set when command execution error occurs 5 CMD Set when an illegal command is received 6 URQ Set by any key press or knob rotation 7 PON Set by power-on

LIA STATUS BYTE (6-29)

bit name usage 0 RESRV Set when a RESRV overload is detected 1 FILTR Set when a FILTR overload is detected 2 OUTPT Set when a OUTPT overload is detected 3 UNLK Set when reference unlock is detected 4 RANGE Set when detection freq crosses 200 Hz 5 TC Set when time constant is changed 6 TRIG Set when unit is triggered 7 PLOT Set when a plot is completed

ERROR STATUS BYTE (6-30)

bit name usage 0 Prn/Plt Err Set when an printing or plotting error occurs 1 Backup Error Set when battery backup fails 2 RAM Error Set when RAM Memory test finds an error 3 Disk Error Set when a disk error occurs 4 ROM Error Set when ROM Memory test finds an error 5 GPIB Error Set when GPIB binary data transfer aborts 6 DSP Error Set when DSP test finds an error 7 Math Error Set when an internal math error occurs

1-10

GETTING STARTED

YOUR FIRST MEASUREMENTS

The sample measurements described in this section are designed to acquaint the first time user with the SR850 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.

The experimental procedures are detailed in two columns. The left column lists the actual steps in the experiment. The right column is an explanation of each step.

Key Types There are two types of front panel keys which will be referred to in this

manual. Hardkeys are those keys with labels printed on them. Their function is determined by the label and does not change. Hardkeys are referenced by brackets like this - [HARDKEY]. The softkeys are the six gray keys along the right edge of the screen. Their function is labelled by a menu box displayed on the screen next to the key. Softkey functions change depending upon the situation. Softkeys will be referred to as the <Soft Key> or simply the Soft Key.

Hardkeys The keypad consists of five groups of hardkeys. The ENTRY keys are

used to enter numeric parameters which have been highlighted by a softkey. The MENU keys select a menu of softkeys. Pressing a menu key will change the menu boxes which are displayed next to the softkeys. Each menu groups together similar parameters and functions. The CONTROL keys start and stop actual data acquisition, select the cursor and toggle the active display. These keys are not in a menu since they are used frequently and while displaying any menu. The SYSTEM keys output the screen to a printer and display help messages. These keys can also be accessed from any menu. The AUTO keys perform auto functions such as Auto Gain and Auto Phase.

Softkeys The SR850 has a menu driven user interface. The 6 softkeys to the right

of the video display have different functions depending upon the information displayed in the menu boxes at the right of the video display. In general, the softkeys have two uses. The first is to toggle a feature on and off or to choose between settings. The second is to highlight a parameter which is then changed using the knob or numeric keypad. In both cases, the softkey selects the parameter which is displayed adjacent to it.

Knob The knob is used to adjust parameters which have been highlighted by a

softkey. Many numeric entry fields may be adjusted with the knob. In addition, many parameters are adjusted only with the knob. These are typically parameters with a limited set of values, such as sensitivity or time constant. In these cases, the parameter is selected by a softkey. The [CURSOR] key will set the knob function to scrolling the cursor within the active chart display.

2-1

Getting Started

2-2

The Basic Lock-in

THE BASIC LOCK-IN

This measurement is designed to use the internal oscillator to explore some of the basic lock-in functions. You will need BNC cables.

Specifically, you will measure the amplitude of the Sine Out at various frequencies, sensitivities, time constants and phase shifts. The "normal" lock-in display will be used throughout this exercise.

1. Disconnect all cables from the lock-in. Turn the power on while holding down the [←] (backspace) key. Wait until the power-on tests are completed.

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

When the power is turned on with the backspace key depressed, the lock-in returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

The display is the "normal" lock-in display. The lock-in setup is displayed across the top of the screen. The sensitivity, reserve, time constant, prefilters and input configuration are all easily visible. Watch how these indicators change as you change parameters. The upper numeric readout and bar graph shows the value of X (Rcosθ) and the lower graph shows the the value of Y (Rsinθ).

The input impedance of the lock-in is 10 MΩ. The Sine Out has an output impedance of 50Ω. Since the Sine Output amplitude is specified into a high impedance load, the output impedance does not affect the amplitude.

The lock-in defaults to the internal oscillator reference set at 1.000 kHz. The reference mode (Intrnl) and frequency are displayed at the bottom of the screen. In this mode, the lock-in generates a synchronous sine output at the internal reference frequency.

3. Press [AUTO PHASE]

4. Press [REF/PHASE]

The sine amplitude is 1.000 Vrms and the sensitivity is 1 V(rms). Since the phase shift of the sine output is very close to zero, the upper display (X) should read close to 1.000 V and the lower display (Y) should read close to 0.000 V.

Automatically adjust the reference phase shift to eliminate any residual phase error. This should set the value of Y to zero.

Display the Reference and Phase menu. The phase shift (displayed in the top menu box) should be close to zero.

2-3

The Basic Lock-in

5. Press the <Rotate 90 deg> softkey.

Press the <deg.> softkey. Use the knob to adjust the phase shift until Y

is zero and X is equal to the positive amplitude.

Press [9] [0] [ENTER]

Press [AUTO PHASE]

6. Press <Ref. Frequency> Press [1] [2] [.] [3] [4] [5] [EXP] [3] [ENTER]

This adds 90° to the reference phase shift. The value of X drops to zero and Y becomes minus the magnitude (-1.000 V).

Highlight the phase shift. The knob can be used to adjust parameters which

are continuous, such as phase, amplitude and frequency. The final phase value should be close to zero again.

Phase shifts can also be entered numerically. The knob is useful for small adjustments while

numeric entry is easier when changing to a precise value or by a large amount.

Use the Auto Phase function to return Y to zero and X to the amplitude.

Highlight the internal reference frequency menu. Enter 12.345 kHz in exponential form. The meas-

ured signal amplitude should stay within 1% of 1 V and the phase shift should stay close to zero (the value of Y should stay close to zero).

Press [1] [0] [0] [0] [ENTER]

7. Press <Sine Output> Use the knob to adjust the amplitude.

Press [.] [0] [1] [ENTER]

8. Press [GAIN/TC] Press [AUTO GAIN]

Parameters can be entered in real or integer form as well. In this case, the frequency is changed to

1.000 kHz. The internal oscillator is crystal synthesized with

25 ppm of frequency error. The frequency can be set with 5 digit or 0.1 mHz resolution, whichever is greater.

Highlight the sine output amplitude. As the amplitude is changed, the measured value

of X should equal the sine output amplitude. The amplitude may be entered numerically also. The sine amplitude can be set from 4 mV to 5 V

rms into high impedance (half the amplitude into a 50 Ω load).

Display the Gain and Time Constant menu. The Auto Gain function will adjust the sensitivity so

2-4

The Basic Lock-in

that the measured magnitude (R) is a sizable percentage of full scale.

9. Press <Sensitivity>

Use the knob to change the sensitivity to 50 mV.

Change the sensitivity back to 20 mV.

10. Press <Time Constant>

Use the knob to change the time constant to 300 µs.

Change the time constant to 3 ms.

11. Press the <Filter db/oct.> softkey until 6 dB/oct

is selected.

Highlight the full scale sensitivity. Parameters which are discrete values, such as

sensitivity and time constant, can only be changed with the knob. Numeric entry is not allowed for these parameters.

Highlight the time constant. The values of X and Y become noisy. This is

because the 2f component of the output (at 2 kHz) is no longer attenuated completely by the low pass filters.

Let's leave the time constant short and change the filter slope.

Parameters which have their available options displayed within the menu box are selected by pressing the corresponding softkey until the desired option is chosen.

The X and Y outputs are somewhat noisy at this short time constant and only 1 pole of low pass filtering.

Press <Filter db/oct.> again to select 12 dB/oct.

Press <Filter db/oct.> twice to select 24 db/oct.

Press <Filter db/oct> again to select 6 db/oct.

12. Press [REF/PHASE]

Press <Ref. Frequency> Press [5] [0] [ENTER]

The outputs are less noisy with 2 poles of filtering.

With 4 poles of low pass filtering, even this short time constant attenuates the 2f component reasonably well and provides steady readings.

Let's leave the filtering short and the outputs noisy for now.

Display the Reference and Phase menu. Highlight the internal reference frequency. Enter 50 Hz for the reference frequency. With a

3 ms time constant and only 6 db/oct of filtering, the output is totally dominated by the 2f component at 50 Hz.

2-5

The Basic Lock-in

13. Press [GAIN/TC] Press <Synchronous> to select <200 Hz.

Display the Gain and Time Constant menu again. This turns on synchronous filtering whenever the detection frequency is below 200 Hz.

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 requiring extremely long time constants.

The outputs are now very quiet and steady, even though the time constant is very short. The response time of the synchronous filter is equal to the period of the detection frequency (20 ms in this case).

This concludes this measurement example. You should have a feeling for the basic operation of the menus, knob and numeric entry. Basic lock-in parameters have been introduced and you should be able to perform simple measurements.

2-6

Displays and Traces

DISPLAYS and TRACES

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 100 mVrms sine wave at 1.000 kHz (the DS345 from SRS will suffice), 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. The displays will be configured to show X, Y, R and θ in bar graph and polar formats. The example Scans and Sweeps demonstrates the use of the chart graph.

1. Disconnect all cables from the lock-in. Turn

the power on while holding down the [←] (backspace) key. Wait until the power-on tests are completed.

2. Turn on the function generator, set the fre-

quency to 1.0000 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 the backspace key depressed, the lock-in returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

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 lock-in defaults to the internal oscillator reference set at 1.000 kHz. The reference mode (Intrnl) and frequency are displayed at the bottom of the screen. In this mode, the internal oscillator sets the detection frequency.

3. Press [REF/PHASE]

The internal oscillator is crystal synthesized so that the actual reference frequency should be very close to the actual generator frequency. The X and Y displays should read values which change very slowly. The lock-in and the generator are not phase locked but they are at the same frequency with some slowly changing phase.

Display the Reference and Phase menu.

2-7

Displays and Traces

Press <Ref. Frequency> Use the knob to change the frequency to

999.80 Hz.

4. Press [DISPLAY/SCALE]

Press <Type/Trace> twice to highlight the displayed trace number.

Highlight the internal oscillator frequency. 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 reference and signal frequency). The X and Y output displays should now oscillate at about 0.2 Hz (the accuracy is determined by the crystals of the generator and the lock-in).

Display the Display and Scale menu. The SR850 collects data in the form of traces.

There are 4 definable traces and only these trace quantities may be displayed. The default definition of these traces is X, Y, R and θ for traces 1, 2, 3 and 4.

The Display Scale menu box shows the display parameters for the Full (screen), Top or Bottom (split screen) displays. In this case, the Top display parameters are shown.

Each display shows one of the data traces. The Top display defaults to showing trace 1 which has a default definition of X. Thus the top bar graph shows the X output.

Use the knob to change the trace number to 3.

Press [AUTO SCALE]

5. Press <Full, Top or Bottom> to select Bottom.

Press <Type/Trace> twice to highlight the displayed trace number.

Use the knob to change the trace number to 4.

Trace 3 has a default definition of R so showing trace 3 on the top graph will display the quantity R.

R is phase independent so it shows a steady value (close to 0.500 V).

[AUTO SCALE] automatically scales the active display. The top display is the active display (as indicated by the inverse trace identifier at the upper left of the display).

To modify the bottom graph, you must display the bottom graph's parameters in the Display Scale menu box. This also makes the bottom display the active display (for autoscaling). The trace indicator (at the upper left of each display) is highlighted on the active display.

The bottom display defaults to trace 2 (Y).

Trace 4 is θ. The phase between the reference and the signal changes by 360° every 5 sec (0.2 Hz difference frequency).

2-8

Displays and Traces

Press [AUTO SCALE]

Press the <± Range> softkey. This is the fifth softkey from the top.

Press [1] [8] [0] [ENTER]

6. Press <Monitor> to select Input/Output.

7. Press <Format> to select Single.

[AUTO SCALE] automatically scales the active display. In this case, the trace data is moving and autoscaling may not do a very satisfactory job.

To manually set the graph scale, you set the range (±) and center value (@). The graph displays a scale equal to the center value plus and minus the range.

In this case, set the bar graph to ±180°. The bar graph should be a linear phase ramp at 0.2 Hz.

The monitor display at the top of the screen monitors either the lock-in settings (sensitivity, time constant, etc.) or the measured lock-in inputs and outputs (X, Y, R, θ and Aux In 1-4).

The Input/Output monitor allows you to see all of the measured quantities, even if they are not shown on the larger displays.

The screen is now setup for a single display. The default display type for the full screen display is a polar graph.

8. Press [REF/PHASE]

Press <Ref. Frequency> Use the knob to adjust the frequency slowly to

try to stop the rotation of the signal vector.

9. Use a BNC cable to connect the TTL SYNC

output from the generator to the Reference Input of the lock-in.

Press <Ref. Source>

The polar graph plots the quantities X and Y on an X-Y axis. The resulting vector has a length equal to the magnitude R and has a phase angle relative to the positive X axis equal to θ. In this case, since the phase is rotating at the difference frequency, the vector rotates at 0.2 Hz.

Display the Reference and Phase menu. Highlight the internal oscillator frequency. As the internal reference frequency gets closer to

the signal frequency, the rotation gets slower and slower. If the frequencies are EXACTLY equal, then the phase is constant.

By using the signal source as the external reference, the lock-in will phase lock its internal oscillator to the signal frequency and the phase will be a constant.

Highlight the reference source.

Use the knob to select External.

Select external reference mode. The lock-in will phase lock to the signal at the Reference Input.

2-9

Displays and Traces

Press <Ref. Slope> to select Rising Edge.

10. Press [DISPLAY/SCALE] Press <Format> to select Up/Down.

11. Press [TRACE/SCAN] Press <1 / 2 / 3 / 4> to select trace 3.

With a TTL reference signal, the slope needs to be set to either rising or falling edge.

The signal vector on the polar graph will not rotate since the phase is a constant. The actual phase depends upon the phase difference between the function output and the sync output from the generator.

The external reference frequency (as measured by the lock-in) is displayed at the bottom of the screen. The reference mode is shown as Ext+ for external TTL rising edge. The LOCK indicator should be on (successfully locked to the external reference).

Display the Display and Scale menu. Change the screen to dual display mode again.

Display the Trace and Scan menu. Trace 3 is defined as R and is displayed on the top

graph. Let's change the definition of trace 3 to something else.

Press the second softkey, next to the trace definition, to highlight the R.

Use the knob to change the A parameter from R to X.

Press the second softkey to highlight the denominator (C) of the trace definition.

Use the knob to change the C parameter from 1 to R.

12. Press [REF/PHASE]

Press [AUTO PHASE]

Traces are defined as A•B/C. The quantities A, B, and C are selected from the various quantities measured by the lock-in.

Trace 3 has now been redefined to be X. The top graph now displays X. The trace definition is shown at the upper left of each graph.

Traces may be defined to be ratios and products of 2 or 3 quantities.

Trace 3 is now defined as X/R and is equal to cosθ, independent of the signal amplitude. The traces can be defined to display the most useful quantities for a given experiment. Trace data may be stored in the data buffers. Scans and chart graphs will be discussed in a later example.

Change the reference phase shift to check that trace 3 displays cosθ.

Automatically adjust the measured phase shift to zero. The top display should show cos0° or 1.

Press <deg.>

Highlight the phase shift.

2-10

Displays and Traces

Using the keypad, enter a phase shift which is 45° greater than the displayed phase shift.

At a measured phase shift of 45°, trace 3 should equal cos45° or 0.707.

This concludes this measurement example. You should have a feeling for the basic operation of the display types and trace definitions.

2-11

Displays and Traces

2-12

Outputs, Offsets and Expands

OUTPUTS, OFFSETS and EXPANDS

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 [←] (backspace) key. Wait until the power-on tests are completed.

2. Connect the Sine Out on the front panel to the

A input using a BNC cable.

When the power is turned on with the backspace key depressed, the lock-in returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

3. Connect the CH1 output on the front panel to

the DVM. Set the DVM to read DC Volts.

The CH1 output defaults to X. The output voltage is simply (X/Sensitivity - Offset)xExpandx10V. In this case, X = 1.000 V, the sensitivity = 1 V, the offset is zero percent and the expand is 1. The output should thus be 10 V or 100% of full scale.

2-13

Outputs, Offsets and Expands

4. Press [REF/PHASE] Press <Sine Output> Press [.] [5] [ENTER]

5. Press [OUTPUT/OFFSET]

Press <Auto Offset>

Display the Reference and Phase menu. Highlight the sine output amplitude. Enter an amplitude of 0.5 V. The top display

should show X=0.5 V and the CH1 output should be 5 V on the DVM.

Display the Output and Offset menu. This menu chooses which measured parameters or traces are output on CH1 and CH2. In addition, the X, Y and R offsets and expands are programmed in this menu.

The Offset & Expand menu box displays the offset and expand of either X, Y or R. In this case, the X offset and expand is displayed. The <X,Y,R> softkey selects the which offset and expand is displayed.

Auto Offset automatically adjusts the Xoffset (or Y or R) such that X (or Y or R) becomes zero. The offset should be about 50% in this case. Offsets are useful for making relative measurements. In analog lock-ins, offsets were generally used to remove DC output errors from the lock-in itself. The SR850 has no DC output errors and the offset is not required for most measurements.

Press <Offset> Press [4] [0] [ENTER]

Press <Expand> Set the expand to 10 using the knob or the

numeric entry keys.

The offset affects both the displayed value of X and any analog output proportional to X. The CH1 output should be zero in this case.

The highlighted OFFST indicator turns on at the bottom left of the top display to indicate that the displayed trace is affected by an offset.

Highlight the X offset. Enter an offset of 40% of full scale. The output off-

sets are a percentage of full scale. The percentage does not change with the sensitivity. The displayed value of X should be 0.100 V (0.5 V - 40% of full scale). The CH1 output voltage is (X/Sensitivity - Offset)xExpandx10V.

CH1 Out = (0.5/1.0 - 0.4)x1x10V = 1 V Highlight the X expand. With an expand of 10, the display has one more

digit of resolution (100.XX mV).

2-14

Outputs, Offsets and Expands

The highlighted EXPD indicator turns on at the bottom left of the top display to indicate that the displayed trace is affected by an expand.

The CH1 output is (X/Sensitivity - Offset)xExpandx10V. In this case, the output voltage is

CH1 Out = (0.5/1.0 - 0.4)x10x10V = 10V The expand allows the output gain to be increased

by up to 256. The output voltage is limited to

10.9 V and any output which tries to be greater will turn on the OUTPT overload indicator at the bottom left of the screen.

With offset and expand, the output voltage gain and offset can be programmed to provide control of feedback signals with the proper bias and gain for a variety of situations.

Offsets do add and subtract from the displayed values while expand increases the resolution of the display.

6. Connect the DVM to the X output on the rear

panel.

7. Connect the DVM to the CH1 output on the

front panel again. Press <Expand> Press [1] [ENTER]

The X and Y outputs on the rear panel always provide voltages proportional to X and Y (with offset and expand). The X output voltage should be 10 V, just like the CH1 output.

The front panel outputs can be configured to output different traces quantities while the rear panel outputs always output X and Y.

Outputs proportional to X and Y (rear panel, CH1 or CH2) have 100 kHz of bandwidth. The CH1 and CH2 outputs, when configured to be proportional to R, θ, or a trace (even a trace defined as X or Y) are updated at 512 Hz and have a 200 Hz bandwidth. It is important to keep this in mind if you use very short time constants.

Let's change CH1 to a trace.

First, set the X expand back to 1.

Press <Offset> Press [0] [ENTER]

Set the X offset back to 0.0%. X Should be 0.500 V again and the CH1 output

2-15

Outputs, Offsets and Expands

should be 5.0 V.

Press <Source>

Use the knob to select Trace1.

8. Press [TRACE/SCAN]

Press the second softkey, next to the trace definition, to highlight the X.

Use the knob to change the numerator from X to 1.

Press the second softkey twice to highlight the denominator (C) of the trace definition.

Use the knob to change the denominator from 1 to X.

Highlight the CH1 source. The CH1 output is proportional to this source.

CH1 can be proportional to X, R, θ, or Trace 1-4. Choose Trace 1. Trace 1 has a default definition of X so the CH1 output should remain 5.0 V (but its bandwidth is only 200 Hz instead of 100 kHz).

Display the Trace and Scan menu. Traces are defined as A•B/C. The quantities A, B,

and C are selected from the various quantities measured by the lock-in.

Trace 1 is defined as X by default. Let's change it to 1/X.

Trace 1 is now 1•1/1 and the top display shows

1.000 Change the denominator.

Trace 1 is now defined as 1/X. The top display shows Trace 1. The trace definition is shown at the upper left of the top display. The trace units are shown at the bottom center of the top display (1/V).

Remember, X was 0.5V. Thus, 1/X is 1/0.5 = 2.0 (1/V). The display should show 2.0 (or very close). Displays use the actual measured quantities to calculate the value of a trace. If X was 5 mV, the value of Trace 1 would be 1/5 mV or 200 (1/V). Traces are calculated using Volts, degrees, and Hz for the units of A, B and C.

The CH1 output voltage is 0.2V. This is because trace output voltages are calculated using the output voltages of the A, B and C quantities rather than their displayed values. In this case, X=0.5V. As an analog output voltage, this would be 5.0 V (1/2 scale of 1V full scale sensitivity). The 1/X output voltage is 1/5.0V or 0.2 V.

See the DC Outputs and Scaling discussion in the Lock-In Basics section for more detailed information.

2-16

Scans and Sweeps

SCANS and SWEEPS

This measurement is designed to use the internal oscillator to explore some of the basic lock-in functions. You will need BNC cables.

Specifically, you will measure the response of the line notch filters by sweeping the internal reference frequency and measuring the sine output. Traces and strip chart displays will be used to record X, Y, R and θ as the signal is swept through the input notch filters.

1. Disconnect all cables from the lock-in. Turn

the power on while holding down the [←] (backspace) key. Wait until the power-on tests are completed.

2. Connect the Sine Out on the front panel to the

A input using a BNC cable.

When the power is turned on with the backspace key depressed, the lock-in returns to its default settings. See the Default Settings list in the Menu section for a complete listing of the settings.

3. Press [INPUT/FILTERS]

Press the <Line Notches> softkey until Both filters are selected.

Display the Input and Filters menu. This menu allows the input configuration to be changed.

With the line notch filters engaged, signal inputs at 60 (50) Hz and 120 (100) Hz are removed. Note that the line filter indicators at the top of the screen are both on.

2-17

Scans and Sweeps

4. Press [DISPLAY/SCALE]

Press the <Type/Trace> softkey to select the Trace number.

Use the knob to select Trace 3.

5. Press [REF/PHASE] Press <Ref. Source> Use the knob to select Internal Sweep.

6. Press <Sweep Menu>

Display the Display and Scale menu. This menu configures and scales the different screen displays and graphs.

Highlight the trace number for the Top Bar graph. The SR850 acquires and displays data in the form of traces. The default definitions for the 4 traces are X, Y, R and θ. These definitions may be changed.

Display the magnitude R (Trace 3) on the top graph.

Display the Reference and Phase menu. Choose internal reference frequency sweep. In

this mode, the SR850 will sweep the internal oscillator from a start to a stop frequency.

Set the sweep start and stop frequencies in this submenu. Sweeps may be linear or logarithmic. In

this case, let's use a linear sweep. Press <Start> Press [4] [0] [ENTER].

Press <Stop> Press [1] [6] [0] [ENTER].

7. Press [GAIN/TC] Press <Time Constant> and use the knob to

select 10 ms. Press <Synchronous> to select <200 Hz.

8. Press [TRACE/SCAN]

Highlight the start frequency. Set the start frequency to 40 Hz. The reference

changes to 40.000 Hz as shown in the frequency readout at the bottom center of the screen.

Highlight the stop frequency. Set the stop frequency to 160 Hz.

Display the Gain and Time Constant menu. Choose a short time constant so that the frequen-

cy can be swept in a reasonably short time. Since the sweep frequencies are below 200 Hz,

you can take advantage of the synchronous filter to remove the 2f component of the output without using a long time constant.

Display the Trace and Scan menu. This menu allows the trace definitions to be changed and the scan (sweep) to be configured. Trace data is sampled and stored in the buffers at the sample rate. Swept parameters (reference frequency in this case) are also updated at the sample rate (immediately after the data is sampled). The scan time

2-18

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