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Re: NE567 tone detector (help)

Hi Bob.

> The book says the freq of operation is defined to be
> 1.1 divided by the product of R x C.

Very close to what National Semiconductor claims, also.

> I'm using 12 k and .047 uF which should give
> about  2000 Hz.  Problem is the PLL runs at 
> about 1200 Hz and the only input that
> triggers it is 600 Hz not 2000 Hz and there are 
> no other false indications.  

How did you measure the VCO frequency?  A scope with
a high-Z probe (hopefully)?

> It simply works solidly at 600 Hz just as advertized, 
> but there are two facts that are inconsistent with
> the book:
> 1) The working freq is off by about 3 (actually almost
> equal to PI, but it would have to be 2*PI to be
> a confusion between radians and frequency)...
> 2)  There is no mention in the book that the trigger
> tone is � the PLL frequency.

For the LM567, it is.  I think the CMOS version runs
the VCO at twice the detection frequency.

> I measured the capacitor and it is right on .047 uF.

Low leakage, too, right?

> The onlly other design components are the non-critical
> bandwidth capacitor (1uF) and the output filter (1 uF).
> Any ideas?

In situations like this, it has been my experience that
what is believed to be the least suspect turns out to be
the heart of the problem!

Those other capacitors might be more critical than you
think!  Try making the bandwidth capacitor smaller (0.1 uf),
and lastly 0.01 uF.  Also check your input level to make
sure it's not too high.  Make sure you AC couple your input
to the chip with a low leakage cap.

> Im supposed to have this satellite finished by Friday!

If changing the bandwidth cap doesn't solve the problem,
and I can scrounge a 567 or two, fill me in on the details,
and I'll see if I can duplicate the situation myself.

Take THAT, Murphy!  :-)

73, de John, KD2BD

Visit John on the Web at:

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