Roy Lewallen wrote:
Asimov wrote:


I never tested a zener when used as varactor but I think these have a
much greater reverse saturation current (even far below breakdown
threshold) and it is this that might cause comparatively more noise
than a conventional diode with a tiny leakage current. Well, at least
that is what the junction noise equations would seem to indicate.


They also have a whale of a lot more capacitance than a conventional
diode. So I don't think it's a fair comparison.

Okay, if we are discussing strange and high capacitance varactors,
what would you think of using a PIN photo diode as a varactor? They
have more chip area per dollar and per package size and capacitance
than many rectifier diodes, and that are made to have low leakage
current and often have low inductance packaging.
For example:
http://rocky.digikey.com/WebLib/Osra...pw34fa_fas.pdf
looks like a nice 15 to 70 pF capacitor.

I guess you would have to keep them in the dark, unless you wanted to
tune them (biased by a high resistance to a fixed voltage) with a
variable intensity light beam.