Hi Paul,

"Paul Keinanen" wrote in message
...
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 :-).

Very true!

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.

No, a lower impedance would be fine. Still, even at 1 ohm, 30dBm is 1.43Vpp
so it's still far from 'small signal.' I do realize that going from 50
ohms - 1 ohm would get me capacitors that were 50 times as large, but on
the other hand the inductors would be 50 times as small!

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.

Thanks, will do.

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.

Sorry, that was poorly worded. A 2:1 capacitance range giving a 1.414:1
tuning range would be fine. (Large tuning ranges are even better. :-) )

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

If you mean 'reasonable sized' (maybe 3.3pF on up) by 'base capacitors,'
that's what I was planning on doing. Initially I was planning on using reed
relays, but having looked at some Hittite switches, I'm now planning on
using a HMC221.

For ~100fF, I was thinking the problem is that you can't reliably switch in
something as small as 100fF in that it's pretty hard to build anything that
small in a repeatable manner. I mean, you can't buy a 100fF capacitor as a
chip cap -- you end up just using a pad of copper on the PCB above a ground
plane, right? Is that feasible? Perhaps if I stuck 4 such 'capacitors' in
series, each one would then be ~400fF. However, at that point I'd
presumably have to use a field solver to figure out the true capacitance
since all those copper pads are going to be pretty close together to one
another and have non-negligible coupling.

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.

I'm not quite following this (I don't know what a 'varactor varacitor is'?).
Something like:

signal -- | | -- || -- gnd

Where | | is the 5pF capacitor and || is the varactor with anode facing
'left'? I'm not following how the 'back to back' configuration works.

Thanks a lot for the help; you've given me a great deal to think about!

---Joel