Keysight Technologies 3458A Guide D'utilisation page 187

FILTER
The filter math operation simulates the output of a single pole, low pass, RC filter.
This allows you to reduce the effects of random noise while preserving long term
trends. The equation is:
The value of DEGREE corresponds to the step response of the low-pass filter. That
is, if 20 is the value of DEGREE, 20 readings are required for the step response to
achieve 63% of its final value. You can achieve slower response or quieter
readings by increasing the value of DEGREE. The actual time constant (R×C) of
the filter can be determined by:
If DEGREE is larger than 10, (R×C) can be approximated by:
For example (using the first equation), if the reading rate is 200 Hz and the
DEGREE is 20, the time constant is:
Keysight 3458A User's Guide
Result = (Previous Result)×(DEGREE-1)/DEGREE + Reading/DEGREE
Where:
Previous Result is initially set to the value of the first reading and thereafter is
set to the result of this FILTER operation.
Reading is any reading.
DEGREE selects the step response of the filter.
1 1
t = - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1
f DEGREE
ln - - - - - - - - - - - - - - - - - - - - - - -
s
DEGREE 1 –
Where:
t = the time constant (R×C)
f = the sampling rate which is: 1/timer interval (when using the TIMER and
s
NRDGS commands) or 1/effective interval {when using the SWEEP command).
If you are not using the TIMER or SWEEP command, refer to "Determining the
Reading Rate" earlier in this chapter.
≈
t (1/f ) × DEGREE
s
1 1
t = - - - - - - - - - - - - - - - - - - - - - - - 1
–
200 20
ln - - - - - - - - - - -
20 1
–
–
= 0.092 Seconds
Making Measurements 4
187