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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 |
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