An oscillator is a device that produces a recurring waveform or oscillating signal, such as a voltage that continuously fluctuates (oscillates) between two levels. Oscillators are generally used as timing or clocking devices in electronic circuits.

A crystal oscillator is simply an oscillator that's made of a piezoelectric crystal, a material that generates a voltage when subjected to mechanical stress, or generates a mechanical force when voltage is applied to it.

If electrodes are plated on opposite faces of a piezo crystal such as quartz and a potential is applied across these electrodes, forces will be exerted on the bound charges within the crystal, making the crystal bend. Removal of the applied voltage will make it return to its original shape, which may generate a voltage in the process. Assuming that the device is properly mounted, such conditions will allow the crystal to behave as an electromechanical system that vibrates as long as it is subjected to proper electrical excitation.

Figure 1.  Photo of crystal oscillators (left) and the symbol for a crystal oscillator (right)

A crystal oscillator behaves like a circuit composed of an inductor, capacitor, and resistor, resonating at a precise frequency, i.e., its electrical oscillation attains maximum amplitude at this resonant frequency. The resonant frequency and the quality factor Q of a crystal oscillator depends on the crystal dimensions, the orientation of the surfaces with respect to its axes, and how the device is mounted.

The quality factor Q of a resonant circuit is a measure of: 1) how fast the response of the circuit falls off as the excitation frequency moves away from the resonant frequency; and 2) how large the amplitude of the response is at the resonant frequency.  A high Q means that the circuit resonates at a higher amplitude and its response falls off more quickly as the oscillation frequency moves away from resonance.

Crystal oscillators can generate frequencies ranging from a few kHz to a few MHz, and Q's ranging from several thousands to several hundred thousands. These exceptionally high Q values and the fact that quartz is extremely stable with respect to time and temperature is the reason for the frequency stability of crystal oscillators.

Crystal oscillators are usually labelled as 'XTAL' on schematic diagrams.

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