> 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 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.

This is beyond a 16F877.  You probably want a DSP.  If you're very clever,
you might be able to squeak by with an 18 series PIC, but I wouldn't attempt
that unless you are pretty comfortable with digital signal processing
algorithms.  Unless this is a very high volume product where every cent of
parts cost counts, I would go for a DSP.  It's not going to cost that much
more and a decent 16 bit multiply/accumulate will probably be very handy.

As you mentioned, FFT is one possibility.  Another approach might be to do a
software PLL to find the noise signal, subtract it out, then look for the
occasional 500mS burst in the result.  Either way, there are some tricky
issues.  Go hire an expert unless you are very sure you're up to it.

How fast does the noise signal change frequency?  How often do the 500mS
burst of the signal to be measured occur?


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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