Stanford Research Systems SR830 Manual page 16

11. Press the [Slope/Oct] key until 6 dB/oct is
selected.
Press [Slope/Oct] again to select 12 dB/oct.
Press [Slope/Oct] twice to select 24 db/oct.
Press [Slope/Oct] again to select 6 db/oct.
12. Press [Freq]
Use the knob to adjust the frequency to
55.0 Hz.
13. Press [Sync Filter]
The Basic Lock-in
Parameters which have only a few values, such as
filter slope, have only a single key which cycles
through all available options. Press the corre-
sponding key until the desired option is indicated
by an led.
The X and Y outputs are somewhat noisy at this
short time constant and only 1 pole of low pass
filtering.
The outputs are less noisy with 2 poles of filtering.
With 4 poles of low pass filtering, even this short
time constant attenuates the 2f component rea-
sonably well and provides steady readings.
Let's leave the filtering short and the outputs noisy
for now.
Show the internal reference frequency on the
Reference display.
At a reference frequency of 55 Hz and a 6 db/oct,
3 ms time constant, the output is totally dominated
by the 2f component at 100 Hz.
This turns on synchronous filtering whenever the
detection frequency is below 200 Hz.
Synchronous filtering effectively removes output
components at multiples of the detection frequen-
cy. 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.
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