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76
Operation
Chop
When the source is Chop, the SR865A will phase lock an external SR540 Optical
Chopper to the SR865A's internal reference frequency. This is achieved by connecting
the f Reference output from the SR540 to the Ref In of the SR865A. But instead of using
this input as the external reference, the SR865A will servo the SR540 to lock the SR540
to the SR865A internal frequency. This control is via a connection from the SR865A 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 SR865A. This is especially useful in experiments
with multiple SR540 choppers. Each SR540 chopper (with its own SR865A) 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 SR865A 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 SR865A Aux Out 4 (rear panel BNC) to the SR540 CONTROL VOLTAGE
input BNC.
SR865A DSP Lock-in Amplifier
and f
, the effective settling time of the SR540 to commanded frequency
ext
int
output (right hand BNC) to the SR865A Ref In BNC.
DIFF
to one of the four data channels. By
ext
Chapter 3
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