Crystal oscillator are electronic oscillating circuits based on the mechanical resonance in a crystal of piezoelectric material in order to generate an electric signal with a certain frequency and good stability. Clock signals are used by processors or for stabilizing the frequency in radio transmitters and receivers.
Crystal oscillators oscillates when a voltage is applied on electrodes on or close to the crystal. Crystal oscillators are manufactured with frequencies from a few kHz to the MHz area. Most crystal oscillators are used in consumer electronics like watches, radios, mobile phones and computer related equipment. More advanced crystal oscillators are used in radio base station, measurement equipment, signal generation, oscilloscopes and network analyzers.
The frequency of a crystal oscillator can either be the fundamental resonant frequency or a multiple of this frequency, a so-called harmonic frequency. Many mechanical resonators have variation of the oscillating frequency, called overtones. Overtones can be used in the oscillator design but are not as precise as the harmonic frequency from the fundamental frequency. High frequency crystal oscillators are often designed to use the third, fifth or seventh overtone. To manufacture high frequency crystals is a challenge because they must be very thin. With a slightly more complicated circuitry the overtone of a thicker crystal can be used instead.
One advantage with crystal oscillators is that they have low phase noise. This make them suitable in telecom applications, test & measurement and research where precise clock references are needed.
Long term, the resonance frequency is affected by the crystal aging. Changes in the environment such as vibration, pressure, humidity and temperature may affect the frequency, but compensations can often be made in the design. The design of TCXO and OCXO oscillators are made so that they give a stable frequency over a wider ambient temperature range. The compensation circuitry may affect the noise level.
VCXO oscillators are used in digital-TVs, data communication hardware, computers and transmitters. In a VCXO, the frequency output can be adjusted with a control voltage, typically between 0 and 3 V. There is also temperature compensated VCXOs sometimes, called TCVCXO. In applications where VCXOs are used, jitter is typically an important parameter. Jitter is related to phase noise but measured in the time domain.
VCO and Synthesizers
A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. In VCO’s for RF frequencies, the oscillating element is typically a ceramic resonator, a SAW resonator or a planar resonator. Together with phase detectors and loop filters, VCO’s are key building blocks in phase-locked loops. In a phase-locked loop, the output frequency is locked to the frequency of a reference input clock. Phase locked loops are used in frequency synthesizer where multiple frequencies are generated from a fixed oscillator frequency reference. Frequency synthesizers provide local oscillator signals in RF up and down conversion.