TheRadioBoard

I'm wondering if a varactor can be used to replace a floating (non-grounded) variable capacitance. Specifically I'm thinking of using it for a phasing capacitor (the bottom variable capacitor) in a phasing crystal filter:



Normally the varactor anode is directly grounded, but I'm thinking if we ground the anode through an RF choke then we can pick off the RF (after it goes through the voltage-variable varactor capacitance) at the anode through a large-value DC blocking cap. Would that work?

EDIT: Unfortunately, thinking some more, it seems this can't work. If the RF signal passes through the varicap on its way to the following load, then it follows that the RF signal will be rectified by the diode, highly undesirable if only a floating variable capacitance is required. Or is there some trick to be able to get a varactor to float without rectification?

A "varactor" is bisased in backward direction to operate!
Thus no (almost no) demodulation will happen.
But this requres that the RF voltage is much lower than the
varicap bias!.
Georg

Well, good. I tried it in the circuit simulator and it seems to be working. In place of the variable capacitor I use a small cap (to set the range of capacitance swing) in series with the diode in series with a large cap (to pass RF and block DC). Anode and cathode each go through a choke to, respectively, ground and the positive control voltage.

This might be a way to eliminate hand capacitance effects on floating variable capacitors.

Try simulating with two identical varicaps connected back to back. You'll get half the C but the rf bias problem goes away. You'll find this method is used in several published varicap-controlled VFO circuits.

73,

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