Stanford Research Systems SR860 Operation Manual page 94

Dsp lock-in amplifier

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Operation

Chop

When the source is Chop, the SR860 will phase lock an external SR540 Optical Chopper

to the SR860's internal reference frequency. This is achieved by connecting the f

Reference output from the SR540 to the Ref In of the SR860. But instead of using this

input as the external reference, the SR860 will servo the SR540 to lock the SR540 to the

SR860 internal frequency. This control is via a connection from the SR860 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 SR860. This is especially useful in experiments

with multiple SR540 choppers. Each SR540 chopper (with its own SR860) 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) external frequency. The SR540 is driven to phase

lock with this internal frequency, but is subject to the mechanical limitations 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

changes can be observed.

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

Both the SR540 and the SR860 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

Connect the SR860 Aux Out 4 (rear panel BNC) to the SR540 CONTROL VOLTAGE

input BNC.

SR860 DSP Lock-in Amplifier

and f

, the effective settling time of the SR540 to commanded frequency

ext

int

output (right hand BNC) to the SR860 Ref In BNC.

DIFF

to one of the four data channels. By

ext

Chapter 3

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