Stanford Research Systems SR830 Manual page 64

Front Panel
of low pass filtering with 100 ms time constant, the synchronous filter,
then two poles of minimum time constant.
Synchronous filtering removes outputs at harmonics of the reference fre-
quency, most commonly 2xf. This is very effective at low reference fre-
quencies since 2xf outputs would require very long time constants to
remove. The synchronous filter does NOT attenuate broadband noise
(except at the harmonic frequencies). The low pass filters remove out-
puts due to noise and interfering signals. See the SR830 Basics section
for a discussion of time constants and filtering.
Note
:
The synchronous filter averages the outputs over a complete period.
Each period is divided into 128 equal time slots. At each slot, the aver-
age over the previous 128 slots is computed and output. This results in
an output rate of 128xf. This output is then smoothed by the two poles of
filtering which follow the synchronous filter.
The settling time of the synchronous filter is one period of the detection
frequency. If the amplitude, frequency, phase, time constant or slope is
changed, then the outputs will settle for one period. These transients are
because the synchronous filter provides a steady output only if the input
is repetitive from period to period. The transient response also depends
upon the time constants of the regular filters. Very short time constants
(<<period) have little effect on the transient response. Longer time con-
stants (<period) can magnify the amplitude of a transient. Much longer
time constants (≥ period) will increase the settling time far beyond a
period.
Use of the Synchronous filter results in a reduction in amplitude resolu-
tion.
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