Crystal units are used as resonators in crystal oscillator circuits and have a highly accurate frequency, tight tolerance and long-term stability. They come in a wide range of frequencies and provide the clock source. Specifications include resonance frequency, resonance mode, load capacitance and series resistance. The mode of oscillation in the resonance frequency operation is specified at the fundamental frequency or the third overtone.
Crystals have two resonance modes, parallel and series. Load capacitance is an important specification when using parallel-resonant oscillation mode. Oscillator circuits can tolerate some degree of series resistance. Metal, ceramic epoxy and plastic package variety for crystals offer excellent environmental and heat resistance. Applications of a crystal are found in mobile phones, clock circuit for microprocessors, GPS clocks, and ISM band radio frequency communications.