Oscillator Oven

I have been working on a few oscillators for a radio project.  The test oscillator is crystal controlled on a carrier frequency of 104.727270 MHz.  This will be multiplied up to a higher frequency.  Unfortunately I forgot what oven temperature I ordered the crystal for.  I think it was around 50C.  I did not want it to be too high as I am using Styrofoam 3/4” thick for the insulating material. The entire oscillator is heated.  The oscillator is a Butler design which uses a series resonant crystal.  The only tuning is from the resonant circuit of the Butler design.  I set the trimmer capacitor for the best oscillation then adjusted the oven heater until I moved it to the marked crystal frequency.   The oscillator has a temperature sensor on it so I can monitor the temperature.  This makes adjusting the heater easier.  The sensor outputs 0.01 volts/degree K. 

 

I now have a stable operation at a sensor reading of 3.292 V which equates to around 56C.  The frequency counter reads 104.727284 MHz.  The counter is locked to 10-Mhz GPS disciplined oscillator to ensure a correct readout.  I am 14-hz off frequency at this point.  The crystal changes about 57-Hz for a 2 degree C temperature change.  This is around 0.54 PPM.  The oven heater resting current is 0.06 Amps.  The heater is nothing more than a power FET attached to the side of the aluminum oscillator housing.  A thermistor on the oscillator PC board in the gate circuit controls the FET.  An external pot in series with the thermistor is used to adjust the temperature.

 

Now I will remember to write down the temperature specification when I order any new crystals.  55 degrees C. works out well for the type of heater and insulating material I am using. 

 

The next test will be to see how well the heater can hold this temperature when I subject the oscillator package to a temperature change.  I need measure the external and internal temperature while the environment changes.  I can do this by using the GPIB interface on my DVM for the on-board sensor and the RS-232 interface on my Radio Shack hand held DVM.  The sensor is a LM324.  I hope to be able to hold +/- 50 hz from room temperature to an ice bath. That would be 2 degrees C temperature variation.