Stanford Research Systems SR810 Manual page 58

Front Panel
FILTER OVLD
[Slope /oct]
[Sync Filter]
Time constant is a low priority parameter. If the sensitivity, dynamic
reserve, filter slope, or expand is changed, and the present time constant
is below the new minimum, the time constant WILL change to the new
minimum. Remember, changing the sensitivity may change the reserve
and thus change the time constant.
The message 'tc chnG' will be displayed to indicate that the time
constant has been changed, either by increasing the detection frequency
above 200 Hz, or by changing the sensitivity, dynamic reserve, filter
slope, or expand.
The time constant also determines the equivalent noise bandwidth
(ENBW) of the low pass filter. This is the measurement bandwidth for X
and Y noise and depends upon the time constant and filter slope. (See
the Noise discussion in the SR810 Basics section.)
The OVLD led in the Time Constant section indicates that the low pass
filters have overloaded. Increase the time constant or filter roll-off, or
decrease the dynamic reserve.
Analog Outputs with Short Time Constants
When using short time constants below 10 ms, the X and Y analog
outputs from the rear panel or the CH1 output configured to output X
should be used. These outputs have a 100 kHz bandwidth and are
accurate even with short time constants. The CH1 output proportional to
the Display (even if X is displayed) is updated at a 512 Hz rate. This
output does not accurately reflect high frequency outputs.
This key selects the low pass filter slope (number of poles). Each pole
contributes 6 dB/oct of roll off. Using a higher slope can decrease the
required time constant and make a measurement faster. The filter slope
affects the minimum time constant (see above). Changing the slope may
change the time constant if the present time constant is shorter than the
minimum time constant at the new filter slope.
Pressing this key selects no synchronous filtering or synchronous
filtering on below 200 Hz. In the second case, the synchronous filter is
switched on whenever the detection frequency decreases below 199.21
Hz and switched off when the detection frequency increases above
203.12 Hz. The detection frequency is the reference frequency times the
harmonic detect number. The SYNC indicator in the CH1 display is
turned on whenever synchronous filtering is active.
When the synchronous filter is on, the phase sensitive detectors (PSD's)
are followed by 2 poles of low pass filtering, the synchronous filter, then
2 more poles of low pass filtering. The low pass filters are set by the time
constant and filter slope. If the filter slope requires less then 4 poles
(<24 dB/oct), then the unused poles are set to a minimum time constant.
The poles which are set by the time constant are the ones closest to the
PSD's. For example, if the time constant is 100 ms with 12 dB/oct slope
and synchronous filtering is on, then the PSD's are followed by two poles
of low pass filtering with 100 ms time constant, the synchronous filter,
then two poles of minimum time constant.
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