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drifting downconverters



Al WB1BQE wrote:

In my opinion, "open loop" (tracking program prediction based)  doppler will not work well for AO-40 telemetry (at least not in this climate.), although it's certainly better than nothing!  
The predictions are accurate enough, but do not take into account the thermal drift of the converters.  (Both my drake, and my 3733 drift substantially when the wind blows, or at night, as the temperature is dropping.

Wayne W9AE replies:

I hardly notice any drift at all from my TSI downconverter which has a 60C PTC thermistor soldered to the crystal.  The frequency is very stable after about 10 minutes if the ambient temperature isn't extremely cold.  I think the thermistor generates enough heat to stabilize the temperature of the entire oscillator circuit, not just the crystal.

The thermistor is powered by 12V, and it doubles the steady-state current drain of the TSI downconverter.  The initial current drain is even higher for a couple seconds until the thermistor warms up.  I disabled the 12V regulator in the TSI downconverter to make it convenient to power via the coax with a 13.8V power supply.  So the thermistor is now the only major heat source in the downconverter.  Someday I may wrap the downconverter in pipe insulation to help the temperature stabilize more easily in cold weather.

Wayne Estes W9AE
Mundelein, IL, USA
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