MODEL SR530 LOCK-IN AMPLIFIER 1290-D Reamwood Avenue Sunnyvale, CA 94089 U.S.A. Phone: (408) 744-9040 • Fax: (408) 744-9049 Email: info@thinkSRS.com • www.thinkSRS.com Copyright © 1997, 2001 Stanford Research Systems, Inc. All Rights Reserved Rev. 2.
Table of Contents Condensed Information SAFETY and Preparation for use Symbols Specifications Front Panel Summary Abridged Command List Status Byte Definition Configuration Switches Guide to Operation Front Panel Signal Inputs Signal Filters Sensitivity Dynamic Reserve Status Indicators Display Select Channel 1 Display R Output Output Channel 1 Rel Channel 1 Offset Channel 1 Expand Channel 1 X (RCOSØ) Output Channel 2 Display Ø Output Output Channel 2 Rel Channel 2 Auto Phase Offset Channel 2 Expand Channel
Circuit Description Introduction Signal Amplifier Current Amplifier Notch Filters Bandpass Filter Reference Oscillator PSD, LP Filters and DC Amplifier Analog Output A/D's D/A's Expand Front Panel Microprocessor Control RS232 Interface GPIB Interface Power Supplies Internal Oscillator 34 34 34 34 34 35 35 36 36 36 36 36 36 37 37 37 37 Calibration and Repair Introduction Multiplier Adjustments Amplifier and Filter Adjustments CMRR Adjustment Line Notch Filter Adjustment 2xLine Notch Filter Adjustment Repai
Safety and Preparation for Use ***CAUTION***: This instrument may be damaged if operated with the LINE VOLTAGE SELECTOR set for the wrong applied ac input-source voltage or if the wrong fuse is installed. LINE VOLTAGE SELECTION FURNISHED ACCESSORIES The SR530 operates from a 100V, 120V, 220V, or 240V nominal ac power source having a line frequency of 50 or 60 Hz.
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SR530 Specification Summary General Power Mechanical Warranty 100, 120, 220, 240 VAC (50/60 Hz); 35 Watts Max 17" x 17" x 5.25" (Rack Mount Included) 16 lbs. Two years parts and labor. Signal Channel Inputs Voltage: Current: Single-ended or True Differential 6 10 Volts/Amp Impedance Voltage: 100 MΩ + 25 pF, ac coupled Full Scale Sensitivity Current: Voltage: 1 kΩ to virtual ground 100 nV (10 nV on expand) to 500 mV Maximum Inputs Current: Voltage: 100 fA to 0.
Acquisition Time Slew Rate Phase Control Phase Noise Phase Drift Phase Error Orthogonality 25 Sec at 1 Hz 6 Sec at 10 Hz 2 Sec at 10 kHz 1 decade per 10 S at 1 kHz 90° shifts Fine shifts in 0.025° steps 0.01° rms at 1 kHz, 100 msec, 12 dB TC 0.1°/°C Less than 1° above 10Hz 90° ± 1° Demodulator Stability Time Constants Offset Harmonic Rej 5 ppm/°C on LOW dynamic reserve 50 ppm/°C on NORM dynamic reserve 500 ppm/°C on HIGH dynamic reserve Pre: 1msec to 100 sec (6 dB/Octave) Post: 1sec, 0.
Front Panel Summary Signal Inputs Single Ended (A), True Differential (A-B), or Current (I) Signal Filters Bandpass: Q-of-5 Auto-tracking filter (In or Out) Line Notch: Q-of-10 Notch Filter at line frequency (In or Out) 2XLine Notch: Q-of-10 Notch Filter at twice line frequency (In or Out) Sensitivity Full scale sensitivity from 100 nV to 500 mV RMS for voltage inputs or from 100 fA to 500 nA RMS for current inputs.
Phase Controls Adjust phase in smoothly accelerating 0.025° steps, or by 90° steps. Press both 90° buttons to zero the phase. Reference LCD Display reference phase setting or reference frequency Time Constants Pre-filter has time constants from 1 mS to 100 S (6 dB/Octave) Post-filter has time constants of 0, 0.1 or 1.0 S (6 dB/Octave) ENBW Equivalent Noise Bandwidth. Specifies the bandwidth when making Noise measurements.
Abridged Command List OX OX 0 OX 1,v OY OY 0 OY 1,v OR OR 0 OR 1,v Return X Offset Status Turn off X Offset Turn on X Offset, v = offset Return Y Offset Status Turn off Y Offset Turn on Y Offset, v = offset Return R Offset Status Turn off R Offset Turn on R Offset, v = offset P Pv Return the Phase Setting Set the Phase to v.
Status Byte Definition Bit Meaning 0 Magnitude too small to calculate phase 1 Command Parameter is out-of-range 2 No detectable reference input 3 PLL is not locked to the reference 4 Signal Overload 5 Auto-offset failed: signal too large 6 SRQ generated 7 Unrecognized or illegal command Configuration Switches There are two banks of 8 switches, SW1 and SW2, located on the rear panel. SW1 sets the GPIB address and SW2 sets the RS232 parameters.
allowable signals at the inputs. The notch frequencies are set at the factory to either 50 Hz or 60 Hz. The user can adjust these frequencies. (See the Maintenance and Repair section for alignment details.) These filters precede the bandpass filter in the signal amplifier. SR510 Guide to Operation Front Panel The front panel has been designed to be almost self-explanatory.
Dynamic Reserve Sensitivity Range LOW NORM HIGH 1 µV through 500 mV 100 nV through 50 mV 100 nV through 5 mV the output, i.e. in the ac amplifier or output time constant. In this case, the dynamic reserve, sensitivity, time constant, or ENBW needs to be adjusted. UNLK indicates that the reference oscillator is not phase locked to the external reference input.
The left hand analog meter always displays the CHANNEL 1 OUTPUT voltage. Accuracy is 2% of full scale. display CH1 X setting output expand? offset? (RCOSØ) X XOFST R R OFST XNOISE X5 X+Xofst Xofst R+Rofst Rofst X noise X5 yes yes yes yes yes no yes yes yes yes yes adjust X+Xofst Xofst X+Xofst X+Xofst X+Xofst(enbw) X+Xofst The CHANNEL 1 LCD display provides a read-out of the displayed parameter in real units.
in magnitude to the selected sensitivity which is in phase with the reference oscillator will generate a 10V output. The output impedance is <1Ω and the output current is limited to 20 mA. to ON) sets the offset to the previously entered value. If an attempt is made to advance the offset value beyond full scale, the ON LED will blink. An offset up to 1.024 times the full scale sensitivity may be entered. When the EXPAND is on, this is 10X the full scale output.
possible. If R is less than 0.5% of full scale, the phase output defaults to zero degrees. The CHANNEL 2 LCD display provides a read-out of the displayed parameter in real units. The scale of the displayed quantity is indicated by the four scale LED's to the right of the display. This readout auto ranges and will reflect the sensitivity added when the EXPAND function is on. When displaying X6, the scale LED's are off and the units are volts.
down, the offset advances in larger and larger increments, the largest increment being 10% of full scale. When the offset is turned OFF the applied offset returns to zero but the offset value is not lost. The next press of the upper offset key (return to ON) sets the offset to the previously entered value. Y (RSINØ) Output The analog output, Y+Yofst, is available at the Y (RSINØ) BNC connector.
going transitions of the reference input. This mode triggers on a negative pulse train or on the falling edges of a TTL type pulse train (remembering that the input is ac coupled). The pulse width must be greater than 1 µS. Time Constant There are two post demodulator low pass filters, labeled PRE and POST. The PRE filter precedes the POST filter in the output amplifier. Each filter provides 6 dB/oct attenuation.
the instrument. All displays return to normal after 3 seconds. should be allowed to approach a steady value before a reading is taken. For the 1 Hz ENBW, this time is on the order of 15 to 30 seconds; for the 10 Hz ENBW, the output stabilizes much faster. The noise output will vary slightly since there will always be noise variations that are slow compared to the bandwidth. Any DC component in the output will not contribute to the noise.
SR530 Guide to Operation Rear Panel AC Power The ac line voltage selector card, line fuse, and line cord receptacle are located in the fuse holder at the left side of the rear panel. See the section, Preparation for Use at the front of this manual for instructions on setting the ac voltage selector and choosing the correct fuse.
the REF OUTPUT on the rear panel to the REF INPUT on the front panel. The REF OUTPUT is a 1 Vrms sine wave. The SINE OUTPUT may be used as the stimulus to the experiment. The SINE OUTPUT can be set to three amplitudes, 1 V, 100 mV, and 10 mV (rms) using the amplitude switch. The output impedance is 600Ω. The AMP CAL screw adjusts the amplitude. 2) If the VCO INPUT is left open, then the oscillator will run at the top of its range (i.e. 10 Hz, 1 KHz, or 100 KHz).
SR530 Guide to Programming An example of a multiple command is: G 5; T 1,4; P 45.10 It is not necessary to wait between commands. The SR530 has a command input buffer of 256 characters and processes the commands in the order received. Likewise, the SR530 has an output buffer (for each interface) of 256 characters. The SR530 Lock-in Amplifier is remotely programmable via both RS232 and GPIB interfaces. It may be used with laboratory computers or simply with a terminal.
The REM LED is on whenever the SR530 is programmed to be in the remote state. DIGITAL DISPLAY. Typing the phase read command, P, will now return the string 45.00 to the terminal. RS232 Echo and No Echo Operation Now read the gain using the sensitivity read command, G. The response should be 24 meaning that the sensitivity is at the 24th setting or 500 mV. Change the sensitivity by typing G19. The sensitivity should now be 10 mV. Check the front panel to make sure this is so.
n 0 1 2 SR530 Command List The leading letters in each command sequence specify the command. The rest of the sequence consists of parameters. Multiple parameters are separated by a comma. Those parameters shown in {} are optional while those without {} are required. The variables m and n represent integers while v represents a real number. Parameters m and n must be expressed in integer format while v may be in integer, real, or floating point format.
23 200 mV 24 500 mV 6 7 8 Note that sensitivity settings below 100 nV are allowed only when a pre-amplifier is connected. 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 H The H command reads the pre-amplifier status. If a pre-amplifier is connected, a "1" is returned, otherwise, a "0" is returned. The H command is a read only command. I {n} If n is included, the I command sets the remotelocal status. If n is absent, the remote-local status is returned.
2 3 4 5 If n is included, then v may be sent also. v is the offset value up to plus or minus full scale in units of volts. For example, to offset half of full scale on the 100 µV sensitivity, v should be "50.0E-6" or an equivalent value. However, if the sensitivity is then changed to 200 µV, the offset is now half of the new full scale or 100 µV. When the sensitivity is changed, the offset is preserved as a constant fraction of full scale rather than as a voltage referred to the input.
X n {,v} n designates one of the 6 general purpose analog ports located on the rear panel. If n is 1,2,3, or 4, the X command will return the voltage on the designated analog input port (X1-X4) in volts. If n is 5 or 6, then v may also be sent. If v is included, the designated analog output port (X5 or X6) will be set to v volts where v has the range -10.238V to +10.238V. If v is absent, the output value of the selected port is returned. On power-up, port X5 is the ratio output.
characters again. mode should be off when not debugging the GPIB interface.) Reset 3) The Z command resets the unit to its default state. The default front panel settings are listed in the DEFAULTS section of the Guide to Operations. In addition, the interface status returns to LOCAL, the SRQ mask is cleared, the RS232 character WAIT interval is set to 6, and the terminating sequence is reset to the proper defaults.
6) command is: Answers are coming back from the SR530 too slowly due to the W6 default setting for the character interval time. Use the W command to speed up the transmission from the SR530. This can cause problems for the GPIB interface if the echo mode is on (switch 6 of SW1). J {n1,n2,n3,n4} where n1, n2, n3, and n4 are decimal values between 0 and 255 corresponding to the ASCII codes of the desired termination characters.
DC1 RL0 Device Clear capability REN,LLO, GTL not implemented. 'I' command sets Remote-Local. Any SRQ generated by the 'no reference, 'unlock', 'overload', and 'auto over-range' conditions will also reset the corresponding bit in the SRQ mask byte. This is to prevent a constant error condition (such as no reference applied to the input) from continually interrupting the controller.
The Lock-in Technique A Measurement Example The Lock-in technique is used to detect and measure very small ac signals. A Lock-in amplifier can make accurate measurements of small signals even when the signals are obscured by noise sources which may be a thousand times larger. Essentially, a lock-in is a filter with an arbitrarily narrow bandwidth which is tuned to the frequency of the signal. Such a filter will reject most unwanted noise to allow the signal to be measured.
Vpsd1 = = The full-scale sensitivity of 100 nV matches the expected signal from our sample. The sensitivity is calibrated to 1%. The instrument's output stability also affects the measurement accuracy. For the required dynamic reserve, the output stability is 0.1%/°C. For a 10°C temperature change we can expect a 1% error.
appears on both the A & 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 5 in 10 , but the CMRR decreases by about 6 dB/octave (20 dB/Decade) starting at 1KHz. Even 5 with a CMRR of 10 , a 10 mV common mode signal behaves like 100nV differential signal. In the first method, the lock-in uses the A input in a 'quasi-differential' mode.
attenuated by 6 dB/octave more. You may wish to use the bandpass filter and select a low dynamic reserve setting in order to achieve a better output stability. Since the processor can only set the bandpass filter's center frequency to within 1% of the reference frequency, this filter can contribute up to 5° of phase shift error and up to 5% of amplitude error when it is used. In addition, the bandpass filter adds a few nanovolts of noise to the front end of the instrument when it is in use.
this picket fence of frequencies would land on some noise source, giving a spurious result. To overcome this difficulty designers employed tuned amplifiers or heterodyning techniques. All of these 'fix-ups' had drawbacks, including phase and amplitude errors, susceptibility to drift, and cardswapping to change frequencies. SR530 Block Diagram Several new concepts are used to simplify the design of SR530 lock-in amplifier.
and dc amplifiers will affect the stability and dynamic reserve of the instrument. The output is most stable when most of the gain is in the ac amplifier, however, high ac gain reduces the dynamic reserve. The Signal Channel The instrument has both current and voltage inputs. The current input is a virtual ground, and the 100 MΩ voltage inputs can be used as singleended or true differential inputs. For the most demanding applications, the user may specify how the system gain is partitioned.
to the left hand transistor but with the opposite sign. Resistors R112 and R110 attenuate the fed back signal from the output of U101 resulting in a differential input, single ended output, fixed gain of 10 amplifier. P101 adjusts the current balance between the two transistors and therefore their gain match and common mode rejection.
phase or quadrature relationship between the two VCO's. Thus, the output of the second VCO can be shifted from -5 to 185 deg from the reference. U204, and U205 are analog switches which select the feedback capacitors for the 5 decades of operation. The two halves of U202 are matched transconductance amplifiers operating as programmable, voltage controlled, current sources which take the place of the normal, frequency setting, resistors.
used to program the band pass filter and the reference oscillator phase shift. One output is subtracted from the lock-in output in U508 to provide a variable offset and another is the rms noise output. The remaining two outputs generate the magnitude and phase output voltages. U419. Analog switch U418 selects the time constant and gain. The full scale output of U418 is 5 volts. The quadrature demodulator and low pass amplifiers are identical to that described above.
or, generates the gate pulse during which reference pulses are counted. Internal Oscillator The internal oscillator is on a small circuit board attached to the rear panel of the instrument. Local regulators, Q1 and Q2, provide power to the board. The VCO input is internally pulled up by R12. This pulls the VCO input to 10V when the VCO input is left open. 2/4 U1 translates the VCO input voltage to provide a negative control voltage to U2, the function generator. P3 adjusts the VCO calibration.
Calibration and Repair minimize the 500 Hz output. Adjust P403 at location C2 to minimize the 30 Hz output. This section details calibration of the instrument. Calibration should be done only by a qualified electronics technician. Now set the both time constants to 1S. Adjust P404 at location F4 to zero the output. This adjustment has a range of 20% of full scale on the HIGH dynamic reserve setting. (2% on NORM and 0.2% on LOW). This zeroes the DC output of Channel 1 on all dynamic reserve ranges.
The CMRR is adjusted by the single turn potentiometer located at A1 under the single hole at the front of the signal shield. (The shield is the aluminum box on the left side of the main board). Using a small screwdriver, carefully adjust the potentiometer to minimize the 100 Hz output on the scope. Set the DISPLAY to R,Ø and the sensitivity to 5µV and minimize the R output on the Channel 1 meter. Replacing the Front-End Transistors Notch Filters 1) Remove the AC power cord from the unit.
Appendix A: Noise Sources and Cures And Others. Other noise sources include flicker noise found in vacuum tubes, and generation and recombination noise found in semiconductors. Noise, random and uncorrelated fluctuations of electronic signals, finds its way into experiments in a variety of ways. Good laboratory practice can reduce noise sources to a manageable level, and the lock-in technique can be used to recover signals which may still be buried in noise.
Inductive Coupling. Here noise couples to the experiment via a magnetic field: Capacitive Coupling. A voltage on a nearby piece of apparatus (or operator) can couple to a detector via a stray capacitance. Although Cstray may be very small, the coupled in noise may still be larger than a weak experimental signal.
Microphonics provides a path for mechanical noise to appear as electrical noise in a circuit or experiment. Consider the simple circuit below: Resistive Coupling (or 'Ground Loops'). Currents through common connections can give rise to noise voltages. The capacitance of a coaxial cable is a function of its geometry so mechanical vibrations will cause the cable capacitance to vary with time.
data, must also be connected correctly at the terminal end. If the terminal responds to a control line, it will believe that the SR530 is not ready to accept data (because the line is not passed in this example) and will therefore not send any data. Appendix B: Introduction to the RS232 The 'RS232' is a standard for bit serial asynchronous data communication.
baud.) The typical data string 5.1270 has 7 characters, requiring 4 msec to be sent. letter 'A', which has the ASCII code 41H (0100 0001), would appear as follows: If a parity option was selected, the parity bit would be sent after the 8th data bit, but before the first stop bit. Stop Bits Generally, selection of 2 stop bits will result in fewer data transmission errors.
Appendix C: Introduction to the GPIB Data Bus: There are eight data lines which use negative logic and pass the bits of each byte in parallel. The IEEE-488 Standard specifies the voltage levels, handshake requirements, timing, hardware details, pinout and connector dimensions for a 16 line, bit parallel bus. Many instruments may be connected in series to communicate over the same cable. Because the bits are passed in parallel, the GPIB is faster than the RS232.
Appendix D: Program Examples All of the program examples which follow do the same thing, only the computer, language, or interface is changed. The programs read the Channel 1 and 2 Outputs and write the results to the computer screen. In addition, the X6 analog output port is ramped from 0 to 10V. Program Example 1: IBM PC, Basic, via RS232 In this example, the IBM PC's ASYNC port (known as COM1: or AUX: to DOS users) will be used to communicate with the SR530.
Program Example 2: IBM PC, Microsoft Fortran v3.3, via RS232 To use these routines, the file 'for232.inc' (also on the SR575 disk) must be 'included' in the FORTRAN source. Machine language routines to interface to the COM1: RS232 port are provided in the file RS232.OBJ found on the SR575 disk. These routines allow for simple interfacing to the SR530 at 19.2 kbaud from FORTRAN programs.
call rxstr(str2) [ 1000 [ 2000 convert string variable into real variable v1 and v2 read (str1,1000) v1 read (str2,1000) v2 format (bn,f10.0) print results to screen write(*,2000) v1, v2 format(′ Channel 1=′,G10.3,3x, ′Channel 2=′,G10.3) [ ramp x6 by 2.5 mV x6 = x6 + .0025 if (x6.gt.10) x6 = 0.0 [ make x6 command string write (str3,3000) x6 format (′x6,′,f7.
Program Example 3: IBM PC, Microsoft C v3.0, via RS232 To use these routines, the large model must be used. Compile with the /AL switch and link with RS232.OBJ. Machine language routines to interface to the COM1: RS232 port are provided in the file RS232.OBJ found on the SR575 disk. These routines allow for simple interfacing to the SR530 at 19.2 kbaud from C programs.
txstr (″q2$″); /* read channel 2 output */ rxstr (str2); /* into str2 */ sscanf (str2, ″%f″, &v2); /* scan str2 for a float variable */ x += 0.0025; /* increment x6 output by 2.5 mV */ if (x >= 10) x = 0; sprintf (str3, ″X6,%f$″, x); /* make x6 command string */ txstr (str3); /* send x6 command */ /* print results to screen */ printf (″Channel 1 = %10.36 Channel 2 = %10.
Program Example 4: IBM PC,Microsoft Basic, via GPIB This program requires the Capital Equipment Corporation GPIB card for the IBM PC or XT. It has firmware in ROM to interface high level languages to the GPIB. In this program, the CEC card's ROM starts at OC0000H, the system controller's address is 21, and the SR530 has been assigned as GPIB address 23. Subroutine calls in Microsoft BASIC are done to memory locations specified by the name of the subroutine.
390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600 610 620 ′ PRINT “CH1 = ″;V1; ″ CH2 =″;V2 ′ X = X + .0025 ′INCREMENT X6 OUTPUT BY 2.
Program Example 5: HP85 via GPIB This program provides an example of an HP85 program using the GPIB interface which could be used to control the lockin amplifier. In this example, the SR530 should be addressed as device #16 by setting the switch bank SW1 per the instructions Page 7. 10 20 30 40 50 60 70 80 90 100 110 x=0 OUTPUT 716 ; ″Q″ ENTER 716 : V1 DISP ″CH1 = ″ : V1 OUTPUT 716 ; ″Q2″ ENTER 716 : V2 DISP ″CH2 = ″ : V2 X = X + .
Documentation This section contains the parts lists and schematics for the SR530 lock-in amplifier. The first digit of any part number can be used to locate the schematic diagram for the part. For example, R415 is located on sheet 4 of the schematic diagrams.
SR530 COMPONENT PARTS LIST Oscillator Board Parts List REF.
SR530 COMPONENT PARTS LIST Main Board Parts List REF.
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