Joan Ilari said:

<SNIP>
> The key point is what you say : "the voltage controlled oscillator
> (VCO) signal gets into the high gain part of the circuit".

I may be having less trouble with the VCO signal getting into the high
gain part of the circuit because I have down converted to my 2500 Hz tone
detect frequency.  I start out with about 100 dB of gain at 60 kHz, and
down convert to 2500 Hz using a bipolar transistor mixer with  L.O.
(Local Oscillator) injection at the emitter.  Of course, there are all
kinds of mixing products; but I filter out the 2500 Hz, and send that to
the LM567.  You (I think) are detecting 20 kHz, and stray effects are
greater at that frequency than at 2500 Hz.

I tend to think that the LM567 is not the best tone detector available,
but it is cheap!  Maybe someone else on the Piclist can suggest a better
performing IC.

Hope this helps.

Jim Albers

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