James :

The key point is what you say : "the voltage controlled oscillator
(VCO) signal gets into the high gain part of the circuit". Following your
suggestion I have tried to feed the 567 with an independent supply unit
(a battery), but SHARING THE GROUND LINE with the existing one. The
result is that I still have the noise in the amplified signal (FYI the
noise has a 20 KHz freq approx.)

I am using a protoboard. Perhaps I have induced voltages ?. The only
thing I realize I can do is to completely isolate the 567 and feed my signal
to it with an optoisolator. Does it make any sense ?

If I cannot overcome this stupid obstacle I will have to think on
another system to detect my beacon-generated sound !

Desperado**3

James R Albers wrote :
>
>  I am currently working on a project using the LM567 for decoding of slow
>  on-off keying (OOK).  I have found that the voltage controlled oscillator
>  (VCO) signal gets into the high gain part of the circuit.  I was able to
>  alleviate this by using a separate 5V three terminal regulator to supply
>  the LM567.  I also had to add a choke from the B+ line to the input of
>  the regulator;  and a decoupling cap.
>
>  In my experience, the LM567 is easily triggered by noise.  I used an
>  opamp integrator after the LM567 to improve the noise immunity.  But I am
>  decoding slow OOK at 2500 Hz.
>

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