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Re: Inquiry



Hi Roy.

> Hi Guys,
> 
> Is there is anyone out there who has built a radio tuning circuit which
> senses the FM radio discriminator voltage as input and provides mic up/down
> key click tuning as output.  This would be for keeping a radio tuned to a
> 9600bps data signal?

Yes, but I've done it through a homebrew modem.  I sample the peak
positive and the peak negative voltage coming from the discriminator,
average the two, and compare it to a reference voltage in a voltage
comparator that toggles an oscillator.  As Doppler shift does it's thing, 
the comparator output switches, causing the oscillator to run and pulse
the receiver lower in frequency through the mic connection.  Since Doppler
shift only acts in one direction, I don't have provisions for pulsing the
receiver higher in frequency.

The reason I sample the peak-to-peak output voltage and compute an
average rather than simply pass the discriminator output voltage through
a low-pass filter is so Doppler tuning will not be affected by any DC
present in the received FSK, which could otherwise throw the tuning off
(although the chances of that happening are probably pretty small).  It
also doesn't require a lot of low-pass filtering that might otherwise
introduce response time delays.

If you're not too concerned about that, you could probably take a quad
OP-AMP, use one section as a low-pass filter, the second section as a
voltage comparator, and the third section as a gated square-wave
oscillator (to pulse the mic connection).  A dual OP-AMP and an LM555
timer should also do the trick.  :-)


73, de John, KD2BD

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