Stanford Research Systems SR830, Manual

Page: 62 / 177

4-9
Front Panel
and the frequency increases above 200 Hz, the time constant WILL
change to 30 s. Decreasing the frequency back below 200 Hz will NOT
change the time constant back to 100 s.
The absolute minimum time constant is 10 µs. The actual minimum time
constant depends upon the filter slope and the DC gain in the low pass
filter (dynamic reserve plus expand). The minimum time constant is only
restricted if the dynamic reserve plus expand is high and the filter slope
is low - not a normal operating situation. The tables below list the mini-
mum time constants for the different filter slopes and gains.
6 dB/oct DC gain (dB) min time constant
<45 10 µs
<55 30 µs
<65 100 µs
<75 300 µs
<85 1 ms
<95 3 ms
<105 10 ms
<115 30 ms
<125 100 ms
<135 300 ms
<145 1 s
<155 3 s
<165 10 s
<175 30 s
12 dB/oct DC gain (dB) min time constant
<55 10 µs
<75 30 µs
<95 100 µs
<115 300 µs
<135 1 ms
<155 3 ms
<175 10 ms
18 dB/oct DC gain (dB) min time constant
<62 10 µs
<92 30 µs
<122 100 µs
<152 300 µs
<182 1 ms
24 dB/oct DC gain (dB) min time constant
<72 10 µs
<112 30 µs
<152 100 µs
<182 300 µs
To use these tables, choose the correct table for the filter slope in use.
Calculate the DC gain by adding the reserve to the expand (expressed in
dB). Find the smallest DC gain entry which is larger than the gain in use.
Read the minimum time constant for this entry. For example, if the slope
is 12 dB/oct, the reserve is 64 dB, and the X expand is 10 (20 dB), then