Hi to everybody !

I am using a pll-based tone decoder LM567 in a sound-based range
finder : I send an accoustic signal of a given frequency, I measure
the time needed to receive the signal and I (try to) compute the
distance based on the time ellapsed between sending the signal
and getting it in the receiver. Up to here the theory.

In practice, I get the signal from an electret microphone, I amplify
it with a couple of transistors and I get a perfect 0-5V signal
in the output of the last transistor. Up to here the good side of
practice. But... (there is always a but....)

When I feed this signal in a LM567, the signal which was pristine clear,
is blurred with noise induced by the 567 (?). Under this circumstance
the tone decoder works but the R value of the oscillator circuit
in the pll is critical : moving veeeeeryyyyy slightly the trimmer
makes the tone decoder losing the signal.

I have tried to put decoupling capacitors everywhere, but the problem
does not disappear. Anybody has any clue ? If I cannot overcome this,
I will have to get rid of 567s ....

Yours faithfully,

Desperado

--------------------------------------------------------------
     Joan Ilari                 e-mail : joan@ilari.org
     Barcelona                   www   : www.ilari.org
     Spain                      Voice  : +34 93 431 96 39

"I've seen things you people wouldn't believe. Attack ships on
 fire off the shoulder of Orion. I watched C-beams glitter in
 the dark near the Tannhauser gate. All those moments will be
 lost in time, like tears in rain. Time to die..."
                                               -Blade Runner-
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