wrote in message news:yvw6g.8938$ZL6.1511@trndny04...
Shure wireless handheld microphone. Spec can be found here if it can be of
any help
http://www.shure.com/pdf/discontinued/L-Series.pdf

Microphone transmit at 180.4MHZ. It was made in 1990. It uses a 20.05MHZ
crystal by MTRON
as far as I can tell like the one uses in Radio Control car, but about
half the size. Can the
frequency be changed to something like 218.5MHZ by just changing the
crystal?
If so, what frequency should I use?

Thanks for any info you can give.


180.4 divided by 20.05, is near as dammit 9, so the crystal oscillator stage
is probably running through two tripler stages ( these are just 'dirty'
amplifiers with appropriate tuned circuits in their collectors or drains, to
pick off the third and then ninth harmonics. ) First stage may well be the
oscillator itself.

In theory, all you need to do is sub the crystal for one at 1/9th the
frequency of the desired output, so for 218.5, you would need a crystal at
24.277 MHz. The oscillator will probably be ok with this comparitively small
change of frequency, but the tuned circuits in the multiplier stages will
need to be adjusted. The first tripler stage will probably be in range, but
the second will be nearly 40 megs off tune, so you may have difficulty
getting it there without altering values in the tuned circuit. Also, if
there is a following RF PA stage, this may well require retuning also,
depending on whether any tuned circuits are just broadband tanks, or
harmonic filters.

Finally, crystals can have many different specifications in their cut
scheme, and load capacitances, depending on whether they are designed for
series or parallel resonance, or overtone use. Crystal cutters understand
what is required if you can let them have a copy of the oscillator
schematic. At this sort of frequency, it will probably be a series crystal
with 30pf loading.

Also be aware that the band that you are proposing moving the mic to, is not
in general, a license-free band for this sort of use, so depending on where
you are in the world, you would probably be contravening telecoms
regulations, and could render yourself liable to prosecution, if you cause
interference to other legitimate band users.

Arfa