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AIDC Downconverter update



Hi all,
I took a couple of hours this past weekend to look into the LO stability 
problems I've encountered on my 3733.  I took the cover off the unit and 
quickly found the LO.  I'm beginning to think there may be issues with the 
PLL.  Some observations:

For those of you who missed my first post, I was experiencing heavy L.O. 
several hundred Hz. (random direction) changes in the course of a few 
seconds.

Observations:
The crystal appears to be extremely temperature sensitive...blowing a fan 
for a second or two can easily lead to 1000hz change.

If the unit is powered off momentarily, the stability seems much better 
(almost a straight line on audio spectrum analysis software).  I've not yet 
determined the length of time where the signal is stable after a reset but 
it seems to be on the order of minutes.  It does not appear that temperature 
is the culprit in the loss of frequency control as only a momentary off/on 
is necessary to stabilize the LO.

Also, after noticing the frequency instability, I took the crystal out and 
put it in the oven at 160 degrees for 15 minutes.  This seemed to have 
helped a little.

Does anyone have any insight with PLLs that have trouble staying locked?  
I've tried several frequency settings (as adjusted by the trimmer cap) but 
seem to get nearly the same result over time.  Is it possible that the 
crystal is just bad and needs to be replaced?

This is using the original crystal that came with the unit (e.g. IF for 2401 
is around 123 Mhz)

Thanks,
Joe - N5PYK
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