OCXO, abbreviation of Oven Controlled crystal (Xtal) Oscillator, is a type of crystal oscillator, in which a 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 (where “XO” is an old abbreviation for “Crystal Oscillator”.) This type of oscillator achieves the highest frequency stability possible with a crystal. They are typically used to control the frequency of radio transmitters, cellular base stations, military communications equipment, and for precision frequency measurement.
OCXOs are normally designed to the highest standards, using high tolerance quartz crystal resonators. In view of the additional oven hardware and high tolerance crystals, they are much more expensive than other forms of crystal oscillator. As with many other crystal based products, OCXOs are available in a widely variation of packages and package styles. The performance levels and costs also need to be considered as these can vary considerably as well.
The resonant frequency of a quartz crystal varies with frequency, even though they are relatively small when compared to other LC resonant circuits, etc. The degree of variation is highly dependent upon the way the crystal is cut during manufacture. The angles of the plane of the blank with reference to the axes of the original crystal determine many of its properties. These include the mode of vibration, the degree of the piezo-electric effect – i.e. its activity, and of course the temperature stability.
A crystal cut known as the AT cut is used in many general purpose applications as it exhibits a good temperature stability profile for many room temperature applications as its profile shows a low change around 20°C.
As most crystal ovens are run at temperatures well above 20°C, the AT cut crystal is not as suitable for use in oven controlled crystal oscillator applications.
In the 1970s other cuts including the SC cut were developed. This Stress Compensated, SC, quartz crystal cut is less sensitive to mechanical and thermal stress giving it a much more suitable performance for OCXO operation. In addition to this it has a lower temperature turning point, LTTP with a very shallow slope in the range 70 to 85°C where many OCXOs run. The reason that the slope is shallow is that the infection temperature is in the region of 90°C as opposed to 25°C in the case of the AT cut crystal.
More recently another cut, termed the IT cut has been developed. This overcomes issues with the requirement for the LTTP to approach the inflection temperature. To address this issue the IT cut, has an upper temperature turning point in the range of 85 to 105°C. However it is not totally ideal because it does not provide the lower level of mechanical stress sensitivity of the SC cut.
There are a lot of OCXO crystal oscillator manufacturers in the world.