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AIDC 3733 rapid freq drift = thermal drift

>From K5GNA:

My experiment with he AIDC 3733 downconverter has been going for the last 11 
hours and the downconverter has stabilized in frequency and of course, 
thermally.  The frequency of the LO is within 100 HZ of where it was for the 
last 10 hours, after it had been on for about an hour.  The drift rate is 
still very slow, cycling and moving only one to three Hertz per count (1 
Hertz gate time) of the counter.  There are none of the rapid, and large, 
frequency excursions as observed when the covers are off and the crystal and 
VCO are subject to air currents.

The easiest way to measure the frequency of the LO with the covers on is by 
injecting a stable signal into the RF input of the downconverter and 
measuring the frequency at the IF with a stable counter.  With a 2401 MHZ 
signal input, the counted output at 123 MHZ will indicate the frequency of 
the LO.  Preferably both the signal generator and counter are locked to the 
same frequency standard.  This test would be very hard to do with a free 
running signal generator.

Unless you want to move the LO frequency beyond the limits of the lock range, 
I can't see the LO circuit needing any major fixes.  The answer to the rapid 
frequency jumping problem that has been observed is thermal stabilization of 
the circuits.  Just leaving the converter powered up with the covers in place 
is one solution.


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