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Wednesday, August 28, 2013

BITX Build Updat #4 VFO Stability

I wasn't quite satisfied with the long-term stability of my VFO.  It seemed like it was drifting about 70 Hz per hour, even after the initial warm-up period.   So, all out of Q-Dope, I went to the local pharmacy in search of clear nail-polish.  (My family was making fun of me.)   I got home and applied several coats of "Sally Hansen No Chip Top Coat Vernis de Protection."   I let it dry a bit, then started watching the frequency counter.  At first results were disappointing.   Drift continued.  Now it seemed to be drifting up!   I cracked the books.  EMRFD has a good chapter on temperature compensation of VFOs, but the process seemed painful.    I wasn't looking forward to it.

I left the VFO on when I went to work, noting the freq as I departed.   Ten hours later I returned, and was delighted to find the frequency almost exactly where I left it!  It may have taken a while for the nail polish to completely dry.    I'll leave it running again today to see how it does.

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  1. Hello Bill,
    I'm neither an expert on VFOs nor on temperature compensation.
    My guess is, that what you observe is a mix of "aging" and temperature effects - before you consider the painful process of compensation,it might help to give the circuit some time to settle or try artificially aging in order to speed up that process.
    Good Luck

  2. Bill,

    You'd have seen the effect of the varnish drying and taking away heat from the core - cooling it down and allowing frequency to change.

    I've messed around with measuring temperature using a method similar to this.


  3. Hi Bill, perhaps try hot-melt glue next toroid. Cheaper than nail varnish and much more "manly"! Hi! It gives the complete assembly a thermal shock and artificially increased aging as it hardens but is best/easiest when it is performed off the pcb, before you solder the inductor in permanently.

    73 de Ted, G4ELM.


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