[hpsdr] Hermes/Anan 10 Drift?

[Mike Monnier]

My Apache Labs Hermes/Anan-10 has, what looks like, frequency instability or
drift that seems to be caused by temperature changes.

I use an external GPS disciplined 10 MHz reference oscillator. The internal
Hermes reference oscillator is not functional due to jumper selection. The 
GPSDO is turned on 24/7 and all diagnostic software shows the unit is
functioning flawlessly. It is an HP 58503A. The disciplined 10 MHz 
oscillator is so close to standard, the difference is measured in parts
per billion. I am quite sure the 10 MHz signal from the GPSDO is 
on the money. I peat this with WWV and watch the phase. 

I first noticed that the receiver calibration was off each time I checked it so
I studied the problem. I use WWV as my calibration standard. Calibration
is off when cold by .100 Hz roughly. To begin I note the external (skin)
temperature of the case using an infrared thermometer. The tests start at switch 
on, skin temperature 66 degrees F and a frequency correction factor of
.9999977100000. I view Phase 2 and adjust the dot rotation until it
slows to a crawl. Almost instantly the calibration starts to shift.

After one hour the skin temp rises to 92 F and correction factor is now
1.000003125.

After two hours skin temp is 101 F and correction factor is 1.00000327.

At hour 3 and 4 the skin temperature stabilizes at 103 F and the correction
factor stays steady at 1.00000327 nominal.

All tests taken with one receiver and no transmit activity. Highest skin temp
area found on the right side of the box facing the front.

I have a small fan for peace of mind when using a 4 watt carrier. No fan was
used during these tests. No air obstructions. No heat source below the radio.

My questions are, what might be causing this problem and can it be corrected?
Thanks!

Mike


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