Temperature stabilization of crystal oscillators for Amateur Radio

"Since my "Hints & Bits" article in June 1994 CHEESE BITS, several homebrewers have asked questions about ovens and thermistors. Which unit is the easiest to use? How are they wired into an existing LO? Does something as simple as a thermistor really work?"

"Why is temperature compensation a concern? The frequency of your transverter's LO can drift many kilohertz when you go roving or during seasonal temperature changes in your basement. Few Amateurs think about the temperature drift of their transverters or commercially made transceivers when they take the equipment out to the field. I measured a 75-KHz shift on the non-ovenized LO of a 10-GHz transverter. Reducing (if not eliminating) the frequency variable when working a weak station is very significant. The easiest, cheapest and fastest way to temperature compensate a crystal is to use a PTC thermistor. The leaded thermistor KC004P is $2.31 from Digi-Key (800-344-4539) and can be added to any crystal circuit in 10 minutes. The unit is connected directly to 12V dc. The Yaesu G9090019 model thermistor comes with its own holder that slides over an HC-25/U crystal. The number for Yaesu USA is 310-404-2700."

"First, quickly unsolder one lead of the thermistor without damaging the metalization. Solder tin the side of the crystal. Solder the flat side of the thermistor to the crystal case. Solder the remaining lead to the +12V line. Solder a small gauge wire from the case of the crystal to dc ground to complete the dc path."

"The thermistor is nominally 50-ohms at 25-deg C, so it draws several hundred milliamps for a short while, then settles down to an idling current less than 30 mA when it reaches its operating temperature. I found the 60-deg C unit to be hot enough, even for the temperature excursions my equipment sees in the attic. Even in the winter the crystal temperature stabilizes after five minutes, instead of drifting for hours. Adding a small styrofoam or insulated cover over the crystal will further stabilize the frequency. Don't get hung up on netting crystals to an exact frequency. Adding a heater will age the crystal in addition to moving its frequency, so allow the transverter to stay on for several days to complete the aging process. You may not be able to pull it back to the original frequency with the crystal trimmer. You can either make a note of the exact frequency and use it that way, or figure out how far the heated crystal moved and order another crystal based on that frequency delta and heater temperature."

"I have added thermistor compensation to several SSB Electronic transverters, all of which adjusted back to the original frequency. The thermistors work great on reference crystal oscillators for PLL sources. Adding a $2.31 thermistor to a DEM no-tune transverter produces a rock-solid LO that won't budge even in the hot sun."

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