Well, I suppose it sort of depends on how fast the constant signal changes,
but...  I wonder if one could sample the continuous signal and synthesize an
equivilant waveform; continue the synthesized waveform based on the measured
signal just prior to the window of interest.  Then you might just subtract
the synthesized signal from the other.  O'Course I havent tried anything
like that so I might just be talking out my backside...

-Dal
----- Original Message -----
From: Douglas Butler <dbutler@IMETRIX.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, July 10, 2001 7:50 AM
Subject: Re: [PIC]: Suggest an approach to this filtering problem?


> If you could synchronize your sampling to the zero crossings of the
> interfering signal it would disappear.  I think filtering in the time
> domain will be tough.
>
> Sherpa Doug
>
> > -----Original Message-----
> > From: adastra [mailto:foster@ADASTRAN.COM]
> > Sent: Monday, July 09, 2001 1:29 PM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: [PIC]: Suggest an approach to this filtering problem?
> >
> >
> > I need to measure the frequency (or period) of a sinewave
> > signal in the
> > range of 1KHz to 10KHz, which is only present for about 0.5 second.
> >
> > The trouble is that there is another signal present ALL of
> > the time that is
> > in the same frequency range and of approximately the same
> > amplitude.  It
> > seems I need to filter out the always-there signal so I can
> > measure the
> > intermittent one.  The exact frequency of the interfering
> > signal is not
> > known, and could be higher or lower than the signal of interest.  It
> > probably must be determined before the filtering can occur,
> > but that part
> > shouldn't be too difficult since the interfering signal is
> > there prior to
> > the appearance of the signal of interest.
> >
> > I realize that if the signal-of-interest happens to be the same as the
> > interfering signal, I probably cannot detect it, but if the
> > filter is narrow
> > enough, I can live with that.
> >
> > I thought I could do this with a couple of analog PLLs ahead
> > of the PIC, but
> > lately I am wondering if I need something more along the
> > lines of a FFT
> > system, or maybe a DSP chip.  I would really like to do
> > everything in a
> > 16F877 if possible.
> >
> > If anyone could point me in the right direction I would
> > appreciate it very
> > much.  Any and all suggestions welcome.
> >
> > Thanks, Foster
> >
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> >
> >
>
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