If its for commercial use it would probably be easiest just to get the
crystals.
Otherwise designing a PLL system is possible - particularly if you want
to learn about them for your own education. But the work involved is not
trivial - probably not worth it for a few old radios. The other issue
will be how the radio achieves FM - if it does it swinging the crystal,
then this will impose another restriction on the PLL. While it is
possible to do (keep loop filter BW below min audio freq) I tried doing
this on a 2M design once - couldnt get it stable. Some radios phase
modulate one of the multipliers, which would avoid this issue.

R

Joel wrote:
I have a couple 6 channel public service radios that are in the 155Mhz
range. Some are synthesized and require cutting diodes to change the
frequency and that works out well.
But some are crystal controlled and with the cost and availability of
crystals it's cost prohibitive to re-crystal these radios. It looks like
these crystals are in the X9 mode so that puts them in the 16-16.5 MHz
range. I am considering building a VCO to inject a signal into the old
crystal socket to get around the crystal issue. Actually I figure a PIC to
program the PLL would be great. I'm just not sure what frequency to inject
into the old crystal socket. Should I build a 16mhz VCO or some harmonic?
Will injecting a signal into a crystal-less oscillator even work? I'm going
to use a ADF4110 PLL and a 16F89 PIC. Any suggestions for the VCO? Schematic
for a 16 MHz VCO?

Joe Loucka AG4QC



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