On May 19, 8:44 pm, Jack Schmidling wrote:
I am in need of zero tc caps for my VFO project but can't seem to find
any info on them. Who makes them? What are they called?

Digikey has some with tc's of 200 ppm but that aint zero.

The also have some that are 0 +/- 500 ppm and that makes no sense.

I need two at 1000 pf.

Thanks,

js

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Look for C0G ceramic caps, or silvered-mica caps. They both have low
temperature coefficients. C0G are normally maximum 30ppm/C. As Uncle
Peter suggested, the inductor will normally have a positive
temperature coefficient of inductance -- it's easy to see why for an
air-core inductor, since it expands and gets bigger as the temperature
increases. You can use C0G for much of the total capacitance, and
combine that with a cap with a negative temperature coefficient--if
you can find one--to get a very low net temperature coefficient of
oscillator frequency. These days, it's fairly common to stabilize a
VFO with a phase or frequency locked loop, so it becomes mainly short-
term stability that you care about in the oscillator design.

Cheers,
Tom