Description
The frequency at which a quartz crystal resonator vibrates depends on its physical dimensions. A change in temperature causes the quartz to expand or contract due to thermal expansion, changing the frequency of the signal produced by the oscillator. Although quartz has a very low coefficient of thermal expansion, temperature changes are still the major cause of frequency variation in crystal oscillators.
The oven is a thermally-insulated enclosure containing the crystal and one or more electrical heating elements. Since other electronic components in the circuit are also vulnerable to temperature drift, usually the entire oscillator circuit is enclosed in the oven. A thermistor temperature sensor in a closed-loop control is used to control the power to the heater and ensure that the oven is maintained at the precise temperature desired. Because the oven operates above ambient temperature, the oscillator usually requires a warm-up period after power has been applied. During this warm-up period, the frequency may not be fully stable.
The temperature selected for the oven is that at which the slope of the crystal's frequency vs. temperature curve is zero, further improving stability. AT- or SC-cut crystals are used. The SC-cut has a wider temperature range over which near-zero temperature coefficient is achieved and thus reduces warmup time. Power transistors are usually used for the heaters instead of resistance heating elements. Their power output is proportional to the current, rather than the square of the current, which linearizes the gain of the control loop.
A common temperature for a crystal oven is 75 °C. but may vary between 30 - 80 °C depending on setup.
Most standard commercial crystals are specified to an environmental temperature of 0 - 70 °C, industrial versions are usually specified to -40 - +85 °C.
Read more about this topic: Crystal Oven
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