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Ask Dr. SETI ®

Chapter 6: Technology

Stabilizing Crystal Oscillators

Dear Dr. SETI:
I built the Down East Microwave SETI down-converter. The local oscillator is only stable to +- 350 hz over a 30 second period. I talked to DEM and they said not to expect better than 100 to 150 Hz stability over that time. That's just not good enough for SETI, is it?
Brian (Project Argus participant)

The Doctor Responds:
No, it's not, but there's a trick! You need to oven your converter's crystal. A proportional temperature controller contains three elements: a heat source, a heat sensor, and a feedback element. All three functions can be performed in a single component, and cheaply, by soldering a positive temperature coefficient (PTC) thermistor to the crystal can, and putting 12 VDC across it. See this photo for details.

Steve at DEM sells the PTC thermistors (60 ohms @ room temperature) for something like a buck. The unit functions as a poor man's crystal oven, thus: The thermistor draws current, and heats up (also heating the crystal can). Its resistance goes up, so it draws less current, and cools down somewhat. Now R goes down, the PTC draws more current, heats up, and so on, eventually reaching thermal equilibrium at about 30 Celsius degrees above room temperature. So what you have is a self regulating proportional temperature controller.

The long term solution will eventually be for each Project Argus participant to use a GPS-disciplined master frequency standard. This will give everybody nearly atomic-clock accuracy, and precise time calibration to boot. But for now, a $1 PTC thermistor will give you an order of magnitude improvement in frequency stability, as compared to running a crystal LO at room temperature.

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