Silver mica capacitors are very good, probably more consistent than NPO
ceramics. NPO ceramics are probable a bit more stable than COG. COG are
tons more stable than general purpose ceramics. The higher the values in
a colpitts circuit for the divider capacitors, the less the effect of
the active device. You'd probably get even better stability with a JFET
in place of the 2N918 though you'd have to change the bias.
You won't need 20 or 30 ma through PIN diodes to switch the crystals,
probably more like a milliamp or two, but I doubt you'll be able to
detect them being better than the 1N4148. There's no way that putting DC
on the crystal will affect it. Its an insulator for DC.
Collins component specs for silver micas should give some ratings on
temperature coefficient.
73, Jerry, K0CQ
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