[GreenKeys] FSK Keyer Information Request

[jhhaynes at earthlink.net]

A lot of modern radios have an FSK input, but it's my understanding that
the FSK input actually shifts an audio tone that is applied to the SSB
generator, so it's the same as using AFSK externally.

With the older radios that have a real VFO you can use either a
diode and capacitor or a varactor diode.  Each is about the same
complexity, so it really doesn't matter.  What you want is a little
capacitor that will move the VFO frequency enough, and then you connect
it through a diode so that back-biasing the diode cuts it out of the
circuit and forward-biasing the diode puts it in.  The interesting
thing is that you can get a variable shift by varying the bias on the
diode.  Some people would bias the diode hard on and adjust the
capacitor to adjust the shift, but most of us left the capacitor alone
and adjusted the voltage into the diode to adjust the shift.  Which
says you could use a fixed capacitor if you wish.  The RF choke is
just to isolate the RF circuit from the DC coming into bias the diode.

Then there was the classical circuit used by the military and commercial
operators.  They ran a 200 KHz oscillator, frequency modulated by a
reactance tube.  This was mixed with the VFO or crystal frequency
to get FSK at the sum or difference frequency.  Advantages are, I guess,
that the shift is independent of the VFO frequency, and maybe the shift
is more linear with voltage, which doesn't matter for RTTY but might
matter for FAX.  Disadvantages are it's another box, quite a big one
in vacuum tube days, and the VFO frequency has to be 200 KHz away from
the intended operating frequency.  And the 200KHz oscillator has its
own drift - they usually put it in an oven.

-- 

jhhaynes at earthlink dot net