[GreenKeys] FSK Keyer Information Request

[AI2Q]

My Ten-Tec Omni-V uses a bipolar transistor to key a varactor on a separate dedicated FSK BFO in the Xmit section. It does not use AFSK in the speech chain.

AI2Q, Alex in Kennebunk, Maine  .-.-.
http://users.adelphia.net/~alexmm/ai2q.htm

  ----- Original Message ----- 
  From: jhhaynes at earthlink.net 
  To: Russmill47 at aol.com 
  Cc: greenkeys at mailman.qth.net 
  Sent: Wednesday, October 13, 2004 11:46 AM
  Subject: Re: [GreenKeys] FSK Keyer Information Request


  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