[HomeBrew] Any PLL experts out there?

[Steven Weber]

Humm, your running the 74AC74's at pretty close to thier max input frequency. I wonder if your just not getting a good clean clocking signal? Marginal drive signal and less then steller power supply decoupling could cause a lot of gitter on the output. 

I'd still take a close look at the loop filter. Have you looked at that with a scope to see how stable it is? 

73, Steve 


  Hi Steven,


  Perhaps I should describe what I observe. When I monitor the PLL output frequency
  with a receiver the other side of the room from the PLL, the note has a significant
  buzz to it. If I tune a kHz or so away from the center frequency the note sounds
  better. I am assuming this is FM since the envelope of the output signal is nice
  and flat.


  I can tune to the output of the 12.3MHz reference (a discrete xtal oscillator) and
  this sounds normal. No hum.


  The tuning step in this design is 5kHz. The VCO is a commercial 85-145MHz
  unit with two stages of 74AC74 dividers following it. I am monitoring the divided-down
  output at 25MHz.


  I have separate 78-series regulators for most of the parts of the circuit. Just added
  a 12V regulator for the VCO - this helps some, as does adding a 47uF capacitor
  on the output of this regulator. Still some noticeable hum, though. I will play
  around with more decoupling in various places to see if it helps.


  Thank you for your help.


  73, ian K3IMW