Tinkering With That Darn Oscillator

It appears the high harmonic output is generated by the last MMIC amp.  Not sure why unless it is sensitive to the impedance seen by the input looking back into the filter.  There is a simple low pass filter after the oscillator buffer.  That filter should have been after the MMIC amp but the amp was an after thought when I designed the PC board.  The harmonics go away when I remove the last output capacitor from the filter.  I then have the second harmonic down 35 DB.  I attempted to replace the first and last capacitor with trimmers to see if I could improve the situation.  Not really, so I left it as it was minus the last capacitor. 

 

I adjusted the oscillator transistor bias resistors to improve the output while maintaining the same 35 db harmonic attenuation.  The oscillator output level is now zero DBm.

 

That still puzzles me as to why I am getting the harmonics.   The MMIC is not being overdriven.  The MMIC is an Agilent ABA-53563 I got from Mouser Electronics.  The 1-DB compression point is +14 DBm output.  I appear to have around -15 DBm going into the MMIC so with a 21.5 DB gain that is below the 1-DB compression point.  I checked my spectrum analyzer calibration against my signal generator.  The spectrum analyzer reading agree with the settings of the generator. 

 

I will leave the oscillator as it is and get back to testing the frequency calibration and oscillator heater.  Something tells me I will be redesigning the PC board moving the filter after the MMIC. 

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.