I've got to make a very clean 10.05944444... MHz VCXO for a redo of one
of my old circuits. I previously used a 10 MHz ceramic resonator, which
was easy enough to push around in frequency. Of course, I have a couple
dozen of those somewhere, but can't find them now that I need them
again. I figured I'd just pull the ones out of the old circuit, but
since I did find a whole bunch of 10 MHz quartz crystals, I'd like to
revisit whether I can push one of those that far with decent results. As
I recall, the results of my previous experiments in doing this were less
than satisfactory, which is why I went with the ceramics.
This would be a change of 60 kHz out of 10 MHz, or 0.6 percent - a
helluva lot for a crystal. The frequency will be exactly phase locked to
a reference. It doesn't need to have extremely high in-circuit Q or
long-term stability - just tunable to that magic number - the PLL will
do the rest. A conventional varicap circuit will provide the VCO-ness,
while the tuning range just needs to be enough to accommodate drift and
the initial setting. The power gain element will be a 74HC04 or 74HC86
section. The PLL reference will be 59.44444... kHz - way above the
necessary loop BW.
Has anyone successfully pushed a quartz crystal that far off, with
reliable (still sort of a sharp resonance) operation and no spurious
modes? Any ideas? If this isn't practical, I'll just go back to the
ceramic resonator (which worked just fine), but I'd like to settle it
once and for all.
Ed
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