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Currently, various kinds of technology are used to stabilize the resonator frequency of crystal oscillators: VCXO, TCXO, OCXO, etc. They have different performance to meet customers' various demands. We illustrate their characteristic as follow so that they can help customers choose correctly accroding to design.
1. VCXO: Voltage Controlled Crystal Oscillator. The output frequency of a VCXO can change with a change of control voltage. The amount of frequency shift with a given voltage change is highly dependent on the oscillator circuit. VCXOs are widely used in telecommunications, phase locked loops, instrumentation, and other electronic equipment when a stable but electrically tunable oscillator is required. The VCXO typically employs a varactor diode to vary the frequency of oscillation by applying control voltage.
2. TCXO: Temperature Compensated Crystal Oscillator. A TCXO adjusts its frequency for temperature variation. As a VCXO is an integral part of a TCXO, the general approach is to use a network of temperature sensors designed to appropriately bias the voltage control of the VCXO for a range of temperatures. The correction voltage is usually applied to a varactor diode in the crystal circuit so that the crystal frequency can be varied by a small amount.
3. OCXO: Oven Controlled Crystal Oscillator. An OCXO offers higher stability and better phase noise specification than other crystal oscillators do. The crystal and other temperature-sensitive components are in a stable oven, which is adjusted to the temperature at which the crystal’s frequency change is the smallest over the temperature range of the oven. Additional circuitry is required to control the oven temperature.
