I just did a design that used a Maxim part:

http://www.maxim-ic.com/datasheet/index.mvp/id/5822

It uses a threshold that is a proportion of the output of a peak
detector.

Neat idea, this part makes it easy. MAX9924.

Bob

On Mon, Apr 9, 2012, at 07:07 AM, V G wrote:
> On Mon, Apr 9, 2012 at 1:04 AM, Sergey Dryga <sergey@dryga.us> wrote:
>=20
> >  Set the trigger level at some low value which you would still consider
> > to be a
> > signal.  Anything above that level will produce "1", below - "0".  This
> > way you
> > eliminate the problem of decay in the amplitude.  For an AC-coupled sig=
nal
> > (through a capacitor) the level can be 1/2 V+.  Essentially, this will =
be a
> > zero-crossing detector.  If the noice is too high, increase the trigger
> > level
> > just above the noise.
> >
>=20
> I thought about this too, but the signal is not a clean sin wave. It's
> unpredictable and has lots of different signals mixed in, so a
> fluctuation
> mid-period from a wave other than the fundamental frequency can cause a
> false trigger.
>=20
> If I had my camera with me, I'd take a picture of the oscilloscope.
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