OCXO Master Clock: Oven-Controlled Crystal Oscillator

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AfterDark OCXO Master Clock:  Oven-Controlled Crystal Oscillator - The ultimate precision timing solution for hi-end audio!  

I would like to share a little basic info on what makes our master clocks so incredibly effective at controlling our audio networks with such laser like precision.

It all starts with the crystal oven. The crystal oven is a temperature-controlled chamber used to maintain the quartz crystal in electronic crystal oscillators at a constant temperature, in-order to prevent changes in the frequency due to variations in ambient temperature. An oscillator of this type is known as an oven-controlled crystal oscillator (OCXO, where "XO" is an old abbreviation for "crystal oscillator".) This type of oscillator achieves the highest frequency stability possible with a crystal. These are typically found in hi-tech manufacturing, medical equipment, cellular base stations, military communications systems and NASA for precision frequency measurement and orientation.

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 circuit 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 to reach its operating temperature.[1] During this warm-up period, the frequency will not have the full rated stability.

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 (Stress-Compensated) crystals are used. The SC-cut has a wider temperature range over which near-zero temperature coefficient is achieved and thus reduces warmup time.[2] 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.[3] but may vary between 30 – 80 °C depending on setup.[4]

Most standard commercial crystals are specified to an environmental temperature of 0 – 70 °C, industrial versions are usually specified to -40 – +85 °C.[5] Crystal ovens are also used in optics. In crystals used for nonlinear optics, the frequency is also sensitive to temperature and thus they require temperature stabilization, especially as the laser beam heats up the crystal. Additionally, fast retuning of the crystal is often employed. For this application, the crystal and the thermistor need to be in very close contact and both must have as low a heat capacity as possible. To avoid breaking the crystal, large temperature variations in short times must be avoided.