re: VFO stability


From: N1OOQ Tom R. @ MRO1 08-Aug-1995 1001 (@)
Date: Tue Aug 08 1995 - 10:22:11 EDT

> The simple Hartley circuit (such as W7EL's Optimized QRP Xcvr) never
> satisfied me -- it never seemed to settle down, with a slow drift downward
> that never stopped. This is after boiling the toroid (which helped),

Yup, I had the same experience with a Hartley. Didn't try boiling the toroid.

> Then I built up the series-tuned Colpitts used by W1FB, a breadboard
> version without a box. At 7 MHz, a toroid inductor (about 6 uH) again
> proved unsatisfactory, the drift being about the same. Using a piece of
> close spaced airwound coil stock, stability improved a lot, with many 1

I tried series-tuned "Clapp" configuration for the 40m superhet I'm building.
I tried two coils: one wound on a T68-6 toroid, the other an air core coil of
about 1 inch diameter, hand wound. Results were worse than the same circuit
with a parallel-tuned tank. I was attempting to get lower drift, as mentioned
by W7ZOI and W1FB in "Solid State Design". I finally gave up on series-tuned,
and went with parallel-tuned Colpitts, which is a great deal better than the
Hartley mentioned above. I don't really know why I got these results for the
Clapp ckt... and now you've confirmed Heyward and DeMaw's findings.

> Better yet, a close-wound wire coil on a 1/2 inch ceramic form (no slug)
> did even better in the above circuit. One run I left it sitting all
> night, and it stayed with a half-step at about 200 Hz audible pitch -- a
> drift of about 10 Hz! Spot checks during the day showed it staying

This I have to try. I've been keeping an eye out for a supply of ceramic-form
coils or just the forms at the flea markets... nothing so far.

-Tom R. N1OOQ

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