The varactor capacitance change is instantaneous -- think of a capacitor
who's plate spacing is dependent on it's voltage. This is why varactors are
used for frequency multiplication -- that capacitance change "squeezes" the
pulse to generate lots of harmonics.

So your oscillation voltage will certainly affect the tuning, and the
forward bias effects will be most unfortunate.

I have modelled varactor diodes by dinking with the area parameter of the
SPICE diode model, but I'm not a pro in that regard and don't know the
"right" way to do it. SPICE is also not the tool for determining the
behavior of nonlinear high-Q circuits (I believe the best tool still
involves FR-4 and lots of cussing).

"James Fenech" wrote in message
...
Hello,

I have a general question regarding tuning diodes:
Does the oscillation voltage alter their capacitance, or is capacitance a
slow changing value - like resistance and PIN diodes?

The reason I ask is that I am spicing (simulation) a VCO I wish to build
(wide range 140-240MHz) and get an oscillation voltage of 20 volts peak
(maybe the Q is too high). This high oscillation voltage would easily
swamp
the tuning voltage, 3-15 volts, and forward bias the diode itself.

If anyone wishs to reply directly they must remove the "xxx" from the
return
address or write to:
james dot fenech at nec dot com dot au

Thanks,
James.