SRS Labs Lock-In Amplifier, SR530 manual Calibration and Repair, Multiplier Adjustments, Cmrr

Calibration and Repair

This section details calibration of the instrument. Calibration should be done only by a qualified electronics technician.

********* WARNING *********

The calibration procedure requires adjusting the instrument with power applied and so there is a risk of personal injury or death by electric shock. Please be careful.

Most of the calibration parameters are determined by a computer aided calibration procedure after burn-in at the factory. These calibration parameters are quite stable and so will not need to be adjusted. Calibration parameters which may need field adjustment are detailed below.

Multiplier Adjustments

On the HIGH dynamic reserve setting, there can be some reference frequency feedthrough. This section describes how to null this unwanted output.

This adjustment requires an oscilloscope and a signal generator which can provide a 500Hz reference signal.

Allow the unit to warm up for about 1 hour.

Reset the unit by turning it off and back on while holding the LOCAL key down.

Select voltage input A and connect a 50 1/2 terminator or shorting plug to the A input BNC connector. Connect the 500 Hz reference signal to the reference input. Set the SENSITIVITY to 1mV and the DYN RES to HIGH. The PRE TIME CONSTANT should be set to 1mS and the POST TIME CONSTANT to NONE. Connect the scope to the CHANNEL 1 OUTPUT on the front panel. Set the scope to 2V/div and 5mS/div. Externally trigger the scope using the reference input signal.

After about 90 seconds, the scope display should show a 500 Hz sine wave on a 30 Hz (500/16 Hz) square wave. Remove the four screws holding the top panel on. Slide the top panel back about half way. Using a small screwdriver, adjust P402 at location D2 to

minimize the 500 Hz output. Adjust P403 at location C2 to minimize the 30 Hz output.

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.

Now connect the scope to the CHANNEL 2 OUTPUT. Set the PRE TIME CONSTANT to 1mS and the POST TIME CONSTANT to NONE. Adjust P1102 to minimize the 500 Hz output. Adjust P1103 to minimize the 30 Hz output. Set both time constants to 1S. Adjust P1104 to zero the output. All three potentiometers are located on the plug-in board in the center of the main circuit board.

Replace the top panel.

Amplifier and Filter Adjustments

This section describes how to adjust the Common Mode Rejection and Line notch filter frequencies. An oscilloscope and a signal generator which can provide an accurate line frequency and twice line frequency sine wave are required.

Allow the unit to warm up for about 1 hour.

Reset the unit by turning it off and back on while holding the LOCAL key down.

Remove the four screws holding down the top panel. Slide the panel back about halfway.

CMRR

Set the reference frequency to 100 Hz. It is convenient to use the SYNC output of the signal generator as the reference input if it is available. Connect the sine output of the signal generator to the A input and set the input selector to A. With the SENSITIVITY at 100mV, adjust the amplitude of the input signal to 100 mV (full scale).

Now set the input selector to A-Band connect the signal to both the A and B inputs. Set the SENSITIVITY to 20µV, the DYN RES to NORM and the BANDPASS filter IN. Connect the scope to the SIGNAL MONITOR output on the rear panel. Set the scope to AC coupled, 0.2V/div, and 10mS/div. Externally trigger the scope using the reference input signal.

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