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Observation: Temperature instability = VCO instability



To: Ward and All;

My previous observations on listening to the beat note are similar to yours.  
They were always with the downconverter mounted on the antenna and after the 
converter had been on for a while.  It was always a very slow drift, with no 
noticeable frequency excursions in the beat note.

I finally got the downconverter reassembled and a hole drilled in the lid 
over the attenuator pad feeding the mixer from the LO.  With a completely 
cutoff BNC connector mounted in the hole in the lid and help from a DEM 2.3 
GHZ LNA, I am able to completely seal the unit and measure the LO frequency 
directly.

So here is what I found:

EQUIPMENT: The counter is an HP 5342 locked to a Rubidium standard, with the 
help of the LNA, the LO signal is -15 dBM into the counter.  The counter is 
not a source of drift.  The downconverter has an original crystal with a 60 
degree PTC thermistor that has heatshrink around it to keep them together.  
This one does not have thermal compound to help heat transfer.  The 
downconverter is externally powered at 16 VDC from an HP power supply; the 
PTC thermistor is powered by the downconverter's internal 7812 at 12 volts.

OBSERVATIONS:  When first tried with no lid, the initial results were the 
same as always, the frequency excursions were abrupt and it wandered 
randomly.  Even with the lids on and the unit completely sealed, the 
excursions were abrupt at first and they wandered randomly. The longer the 
unit was on -- with heat from its own 3 terminal regular and direct heat on 
the crystal, the more the unit stabilized.  As of now, after about 45 
minutes, it appears that he unit has reached some thermal equilibrium.  The 
frequency is essentially cycling over a range of about 10 Hertz within the 
time frame of a couple of minutes, at about one Hertz per second. 

CONCLUSIONS: With the lids off, the VCO and crystal oscillator are subject to 
thermal drift which cause abrupt changes in frequency.  With the unit 
completely sealed and temperature stabilized, it is showing only a small 
thermal drift with no major frequency excursions.  By itself, the 
downconverter will stabilize thermally, to some extent, on its own, I am 
helping the crystal stabilize with the PTC thermistor at 60 degrees C.  

I hope these results will be duplicated and reported.

TNX & 73,

Bob Seydler
K5GNA
k5gna@aol.com

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