FM25/FM100 Frequency Adjustment Mod

                  -by David Schmidt, DSchmidt Technologies

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Do you own a frequency counter? If so, have you checked the transmitting
frequency of your FM25/FM100? Chances are that the frequency that it is
transmitting on is not very close to what it is supposed to be! My own FM25
when set to transmit on 100.0Mhz (for testing purposes) actually transmitted
on 100.0243Mhz!

The digital display on the FM100 only shows the frequency that the PLL is
supposed to be set to. It does not actually measure your transmitting
frequency so it could be off and you wouldn't know it!
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Though the FM25/FM100 is a crystal clocked, PLL locked transmitter, the
output frequency is only as accurate as the 4.00Mhz reference frequency. If
your reference frequency is off, your transmitting frequency will be also.

What you need to do is to 'trim' the reference oscillator so that your
transmitting frequency ends up being dead on target. You do this by changing
the load across the crystal as follows.

   * Find out what value you were shipped for C1 of your FM25. If you have
     an FM100, the capacitor you need to check is C2.
   * Take this value, subtract about 10pF from it and go out and purchase
     another ceramic disc capacitor that is close to this value. (example,
     if C1 is 22pF, go out and buy a 10pF cap. If C1 is 47pF, go out and buy
     a 33pF cap.)
   * Go buy the smallest (physically) trimmer capacitor that you can find
     with a range that is close to 2.0-30pF.
   * Remove the original C1 capacitor on your FM25 (C2 for those of you with
     the FM100) and install the new ceramic disc capacitor.
   * Flip your circuit board over and solder the trimmer capacitor to the
     same pads that C1 on the FM25 (C2 for the FM100) is soldered to. This
     will place the trimmer cap in parallel with the new capacitor. Trim the
     leads of the adjustable cap so that once it is installed, it is low
     enough to still fit within the case.
   * Set your FM25/FM100 to some arbitrary frequency (like 100.0Mhz) and
     turn it on.
   * Turn on your frequency counter and read the frequency you are
     transmitting on.
   * Adjust the trimmer cap until you are dead on target (100.000000Mhz) or
     as close as possible. Do not play an audio into the FM25/FM100 when you
     are making your adjustment. Change the gate time of your frequency
     counter for the best resolution.
   * Button it all back up!



That's all there is to it! If you wish you can drill a small hole in the
bottom on your FM25 case right where the trimmer is so you can 'tweak' the
frequency in the future without opening the unit. The same can probably be
done with the FM100, but I don't own one to be able to check.

The above component value choices are not critical. What you want to end up
with is a combination that will let you vary the capacitance of C1 on the
FM25 (C2 on the FM100) +/- 10pF from the value that was originally shipped.
If your original cap value is 27pF, and you buy a 10pF cap and a 5-40pF
trimmer, you now have a combination that will give you 15-45pF. This is
close enough.

For those of you that want to perform the mod above but don't know where to
get the parts, I am in the process of obtaining parts to resale. Including
instruction sheet, the target price is $5.00 +$1.00 shipping. Stay tuned to
it release announcement!
Dave- DSchmidt Technologies
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dschmidt ( dschmidt@silcom.com) Page updated Nov 13, 1997