On Sat, 30 Oct 2004 01:22:51 -0700, "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.

Perhaps not tuning range, but if there are strong unwanted signal, you
are going to get a lot of unwanted mixing products (this is basically
a varactor multiplier :-).

Is there a specific reason for runnig the filter at 50 ohms (as it
would appear from your figures). Why not design your filter for, say,
1-10 ohms and the voltage range would be reduced.

I'm told that there are 'high voltage'
varactor diodes out there; does anyone have a recommended source?

You might investigate any ordinary small signal signal diodes (such as
1N4148or SMD equivalents) with sufficient breakdown voltages and look
for their reverse voltage vs. capacitance curve.

This is at 500MHz and the nominal component values are ~3pF. If I could
obtain a 2:1 tuning range, I'd be happy.

That would require a 4:1 capacitance range.

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.

If you can switch in (using swithing diodes or reed relays) fixed
capacitances as base capacitances, this should not be a problem.

Put a small (perhaps 5 pF) fixed capacitor in series with a big
(perhaps even 5..50 pF) varactor and you can generate a moderate
tuning range. Using some back to back configuration with anodes at
ground potential and varactor varacitors at the tuning voltage, you
can also reduce the IMD products.

Paul OH3LWR