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A crystal oscillator is an electronic oscillator that makes use of crystal as a frequency selective element to obtain an inverse piezoelectric effect. It makes use of the mechanical resonance of the vibrating crystal, which has piezoelectric properties, in order to obtain an electric signal with a high-precision frequency. Crystal oscillators are considered superior to ceramic resonators as they have higher stability, higher quality, lower cost and are smaller in size.
Crystal oscillators are a prime example of fixed-frequency oscillators where accuracy and stability are the most important considerations. They usually use the same circuits as other types of oscillators, with the difference being the crystal replacing the tuned circuit. In crystal oscillators, the crystal vibrates as a resonator and the resulting frequency determines the oscillation frequency. In other words, the crystal acts like a circuit having an inductor, resistor and capacitor with precise resonant frequency. In some cases, in order to have better thermal stability for the crystal oscillator, temperature compensation is applied.
There are many benefits in using crystal oscillators in digital applications. Crystals can be manufactured to have a wide range of specific frequencies. Compared to tuned circuits, crystals have a high Q-factor, better temperature stability and much better frequency stability than RC oscillators or LC oscillators. Crystals used in crystal oscillators exhibit only very low phase noise.
Crystal oscillators are used mainly in digital integrated circuits for providing a stable clock signal and in specific applications which require high-frequency reference.