The simple packaged crystal oscillator (SPXO) is produced without temperature control or compensation. Crystals are formed using a method called AT-cut for each unit. The SPXO frequency-temperature characteristic is plotted as a cubic plane curve. SPXOs are used as reference oscillators for measuring devices and communications devices. SPXOs are also used in the clocks for office equipment, information terminals and computers.
The temperature compensated crystal oscillator (TCXO) is produced with temperature-compensated circuits designed to withstand a greater range of temperature than the SPXO. These oscillators are used in conjunction with temperature-sensitive devices using microchips with tens of thousands of transistors, called single-chip large scale integration. These oscillating crystals are small, require little power and "start up" rapidly. These oscillators are used in mobile telephones, synthesizers, precision measuring instruments and global positioning systems.
The voltage controlled crystal oscillator (VCXO) is made with variable-capacitance diodes, giving it a the ability to function on a wide frequency range, controlled by external voltage changes. The frequency bands available to the VCXO range from 2 megahertz to 780 megahertz. The VCXO's temperature characteristics are similar to the SPXO, as is its cubic plane curve wave plot. Voltage controlled crystal oscillators are used in synchronization devices and the demodulation of switchboards.
The oven controlled crystal oscillator (OCXO) has the best temperature characteristics of any crystal oscillator. Its temperature is kept constant in a housing that includes a thermostatic device called an "oven." These are considered the most stable of the four main types of crystal oscillators. Upper and lower operating ranges are closely controlled through the interplay of the "oven" and the power input. The OCXO is most suitable for precision equipment in highly controlled environments, like spectrum analyzers.