RE: VXO or VFO???

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From: Harry Chase (Harry_Chase@smtpgwy.windata.com)
Date: Mon Jul 24 1995 - 15:39:49 EDT


I remember that the CB'ers in the 1970's / early 80's, with the old 23
channel crystal rigs used to modify the "clarifier" in the thing to
tune inbetween channels. This was usually done by adding a tunable
inductor to the crystal osc. circuit. Some variations were around,
using resonant LC circuits in series with the crystal, etc., but some
of these could tune 2 or 3 channels down and maybe one up (up to 40
KHz) from the nominal xtal frequency. In a few cases, with the LC
circuit added, you could pull it so far it would "break away" from the
crystal and become a VFO -- then it zipped thru a dozen channels and
drifted... With a spectrum analyzer, years later, I built up one of
these circuits and watched the phase noise slowly increase as I pulled
the crystal more and more. The Hz/turn of the coil was very
nonlinear, suddenly getting MUCH more sensitive just before it broke
away.
          A variable cap or a varactor with a fixed coil will also
work.
          All of these circuits work a lot better with fundamental
mode rocks than with overtones, and sometimes the added tuned circuit
in the oscillator will force the crystal to "jump" to the 3rd overtone
as you push the tuning. They are tricky because its sensitive to the
type of crystal package, the quality of the caps and coils, and
variations from one crystal to another, even out of the same boxful.
     --A counter , a scope, or a spectrum analyzer is recommended when
tuning these things the first time.

Harry
WA1VVH


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