At 1/5/2007 07:47 AM, you wrote: > > > >Mention was made of how to deal with compensation. I've built ovens of > >various types, and even used PTC thermistors soldered directly to the case > >of the crystal as a heater, with good results. I think Bob NO6B did the > >same - you QRV Bob?
Yes, I've been soldering 50 °C PTC thermistors to crystal cases for some time. On my last project using a rather unstable crystal I discovered that the PTC thermistors don't provide as much thermal stability as I thought (transition not sharp enough?), so I built up a feedback controller based on a design out of an old National Semiconductor databook using a dual op amp. I used thermal epoxy to mount a 51 ohm heater resistor & LM34 temperature sensor onto the crystal. The circuit is able to hold the temperature at the LM34 within +/- 0.2 °F. Now the only thermal drift I see is due to the temperature effect on the oscillator transistor on the exciter board. Bob NO6B
