A crystal oscillator uses the mechanical resonance of a vibrating piezoelectric material to generate a very stable clock signal. The frequency is usually used to provide a stable clock signal or to keep track of time; hence, crystal oscillators are widely used in RF and digital circuits.
Crystals are available from various vendors in a variety of shapes and sizes and can vary widely in performance and specifications. Understanding the parameters and the oscillator circuit is essential for a robust application stable over variations in temperature, humidity, power supply, and process.
All physical objects have a natural frequency of vibration, where the vibrating frequency is determined by its shape, size, elasticity, and speed of sound in the material. Piezoelectric material distorts when an electric field is applied and generates an electric field when it returns to its original shape. The most common piezoelectric material used in electronic circuits is a quartz crystal, but ceramic resonators are also used – usually in low-cost or less timing critical applications. 32 KHz (32768 Hz) crystals are usually cut in the shape of a tuning fork and very precise frequencies can be established.
