You might consider putting varactors or switched capacitors across only
part of the inductor, i.e., a tap. The voltage will be lower and the
required capacitance higher.

You might also consider ways to change the inductance instead of the
capacitance.

Q, temperature stability, repeatability, and perhaps other factors will
play a big role in determining which way is best, or acceptable.

Roy Lewallen, W7EL

Joel Kolstad wrote:

I'm looking for suggestions on how one might go about changing the effective
capacitance of a resonant circuit (it's part of a filter) in order to change
the center frequency. The tricky part is that there's ~30dBm (10Vpp) RF
running around, so with standard varactor diodes the RF becomes 'the bias'
and the tuning is destroyed. I'm told that there are 'high voltage'
varactor diodes out there; does anyone have a recommended source?

This is at 500MHz and the nominal component values are ~3pF. If I could
obtain a 2:1 tuning range, I'd be happy. I need perhaps 30 steps within
that range (3-6pF), and (doing the calculations) each step changes the
capacitance by little more than about 100fF to start with -- so I'm thinking
that switching physical capacitors into and out of the circuit is out of the
question here.

Thoughts?

Thanks in advance,
---Joel Kolstad