Along those lines of thought, since we're just interested in the
frequencies, why not stick a zero crossing detector and count pulses with
the pic against a timer to get a frequency.  Subtract the current pulse
block count from that of .5 seconds ago, ifany significant increase from
zero --there's your intermittant signal freq. Again, your continuous signal
can't change significantly fast or this'd frag.

!???
-Dal
----- Original Message -----
From: Dal Wheeler <dwheeler@insightek.com>
To: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, July 10, 2001 12:09 PM
Subject: Re: [PIC]: Suggest an approach to this filtering problem?


> 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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> >
>

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