Hi,

I am working on building a VCO for FSK with 850 Hz separation between
mark and space. I am building it using a 16.000 MHz 32pf parallel load
crystal. I am pulling the crystal via a varactor to implement the FSK.
However, the amplitude between mark and space is an unacceptable 10 dB
with about 700 Hz of shift. Any ideas on how to pull the crystal
without changing the amplitude of the mark and space frequencies
considerably?

A bit about my circuit: have an 8 - 50 pF tuning cap across pins 6 and
7; a 22pF cap shunts pin 7. The crystal shunts pin 6. A 100nF DC block
cap separates pin 6 from the 8 - 29 pF (3 to 10 V reverse bias)
varactor. A 1mH inductor isolates AC from the varactor biasing.

I tried adding 1uH of inductance in series with the crystal. This moved
the oscillation frequency up, but the amplitude problem remained. I
tried putting the varactor across the tuning cap. This configuration
exhibited the same amplitude shift problem, but shifted the frequency
up rather than down. I tried combining the two shifts via two varactors
and a JFET to invert the biases, one across the crystal (as originally)
and one across the tuning cap. This didn't work well (very noisy
spectrum).

Any ideas as to what could be the problem and how to fix it?

Cheers,
Chris