[SI-LIST] Re: DC data

One other possibility, just in case modifying the hardware is out of the
question:   If your noisy waveform is being handled in software, you can
filter it in software.

A symmetric FIR (Finite Impulse Response digital filter) low pass filter
with an odd number of taps (so that there is a tap at exactly the center of
the symmetric impulse response shape) has flat group delay, that is,
exactly the same time delay at all frequencies, which can be compensated
for easily. That can be implemented in software.
The downsides of filtering in software are:  losing (number of taps - 1)
points, half on each end of the waveform;  and filtering in software takes
time - filtering in hardware doesn't (actually, there is delay through any
hardware filter but it's probably negligible).

A FIR lowpass filter can be designed starting with a sin(x)/x shape and
then windowing that with some shape that makes it tail off to zero sooner
than infinity (after perhaps 5 ripples). That can work quite well, the
result is well behaved.  Or, it can be algorithmically designed to meet a
least squared error (in the frequency domain) criterion, or - my favorite -
using the Remes exchange algorithm developed by Parks and McLellan, which
is a Chebyshev approximation (minimizes maximum error.  Result is an
equiripple filter, with ripple in both the pass band and the stop band).

One good little book on digital filters, although somewhat old, is "Digital
Filter Design" by T.W. Parks and C.S. Burrus.  Not surprisingly it has a
good presentation of the Parks - McLellan algorithm.  That book has example
programs.  Also, there are good filter design programs available.

--- Joe S.
LeCroy Corp

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