Yes, the RF generally does cause nearly instantaneous voltage changes,
and therefore capacitance changes, across the diode. Spice should
handle this properly. You need to lower that RF voltage! In
addition, the average capacitance will depend on the amplitude of the
RF because of the nonlinear nature of the capacitance versus voltage,
so it's important to keep the RF level reasonably constant, for
stability. It's fairly common in covering a wide range, especially if
you also want low phase noise, to break it up into subranges which
might be selected with switching or PIN diodes. Of course, it's also
critical to keep the control voltage "quiet" because at 10MHz/volt or
so, a millivolt will give you 10kHz of frequency deviation.

If you look up other uses for varactor diodes, you'll find that the
characteristic you are asking about is used to make efficient
frequency multipliers, and even mixers and amplifiers. (Search for
"parametric amplifier.")

Cheers,
Tom


In message , James French wrote:
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.