subject: Ovenized-controlled Crystal Oscillators For Accurate Frequency Needs [print this page] A crystal oven is a temperature controlled chamber which is used to keep the temperature of quartz crystal in electronic crystal oscillator constant. It reduces the possibilities of changes in the frequency in ambient temperature. Such oscillator is referred to Oven-Controlled Crystal Oscillator or OCXOs. These oscillators are most commonly used where the highest stability needs to be fulfilled. They are usually used in the equipments where prcised frequency require to be maintained such as radio transmitters, cellular base stations, military communication equipments, satellite, etc.
Quartz crystal resonator vibrates at a particular frequency which is determined by its physical dimension. A change in temperature causes the quartz to expand or contract, thereby changing the frequency of the signal produced by the oscillator. Although quartz has a very low coefficient of thermal expansion, temperature changes are still one of the major causes of frequency variation in crystal oscillators. To prevent any frequency variations, crystal oven is used. It is a thermally-insulated enclosure containing the crystal and one or more electrical heating elements.
Usually the entire oscillator circuit is enclosed in the circuit because the other electronic components included in the circuits are also vulnerable to temperature drift. To enhance the stability, the temperature selected at the oven is that at which the slope of the crystal's frequency vs. temperature curve is zero (AT or SC cut crystals are used). The SC-cut features wider temperature range over which near temperature coefficient is achieved thus reducing the warm-up time. Instead of using resistance heating elements, power transistors are used as heaters. Depending upon the set up of the crystal oven, the common temperature may vary however the standard temperature is taken to be 75C.
OCXOs require more power than oscillators that run at ambient temperature. Therefore, these oscillators require heater to run them which resulted in their larger size. Because of the larger size, these oscillators are not used in battery powered or miniature applications. However, the overall stability of OCXOs is far superior to that of other crystal oscillators. There are several Oven-Controlled Crystal Oscillator manufactures in the market that can help you to select or design the right OCXOs for your application. Choosing oscillators can prove to be quite difficult, therefore visiting the right oscillator manufacturer who can guide you and suggest the pros and cons completely is quite essential for the success of your application.
