[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Setting the 3733 exactly on frequency

Hello to all.  I have noticed some of you have not been able to set the
AIDC-3733 crystal frequency exactly so that 2400MHz = 144MHz.  The
tiny trimmer does not have enough range, and some of you are setting
the trimmer so that 2400=144.1 or 144.2MHz.   This is fine and works
well, but there is a simple way to set it exactly.

A few of you have tried padding the crystal with a small capacitor,
but this is the wrong approach.  The converter uses low-side LO
injection. The desired 144MHz output is the difference between the
2400MHz signal and the 2256MHz LO from the PLL.  This means a LOWER
crystal frequency results in a HIGHER dial reading on 2M.  Therefore,
to calibrate the converter, we need to RAISE the crystal frequency, not
lower it with additional capacitance in the crystal circuit.

The tiny little adjustable trimmer capacitor is the culprit.  It's
minimum capacitance is too high, and makes the crystal oscillate
700 or 800 Hz too low.  The VCO phase-locks at 256 times the crystal
frequency, resulting in a LO signal about 100-200KHz low.

The fix is simple, and I did one in about five minutes.  Using a great
deal of care, and the smallest soldering iron tip you can find, remove
the trimmer capacitor.  Alternately, you can crush the part with a
small needle-nose pliers.  If you measure the crystal frequency with the
trimmer removed, you will find the crystal frequency has moved upward
nicely.  Now, it is a little too high, so we need to bring it down.
I used a "gimmick" capacitor, formed from two 3/4" pieces of enameled
coil wire, twisted together loosely, soldered in place of the removed
trim cap.  Be sure to leave the enamel on the wires except where you
solder them in!

You can zero in on very close calibration by the amount of twisting
together of the gimmick capacitor wires.  Bend the ends of the wires
down towards the board so the cover goes on without hitting them or
upsetting your calibration.

Best regards,  John  W5EME
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org