Stanford Research Systems SR810 Manual page 16

The Basic Lock-in
11. Press [Sync Filter] 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 using 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 (18 ms in this case).
This concludes this measurement example. You
should have a feeling for the basic operation of the
front panel. Basic lock-in parameters have been
introduced and you should be able to perform simple
measurements.
2-4