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A quartz oscillator is an electronic circuit which generates a signal at a frequency determined by a quartz crystal formed from silicon and oxygen. The piezoelectric property of quartz makes it a very good resonator from tens of Kilohertz (KHz) to hundreds of Megahertz (MHz). A quartz crystal oscillator is typically designed to operate at a single precise frequency only. These oscillators are used to create stable frequencies in cell phones, Global Positioning System (GPS) receivers and two-way radios. They are also used for precise timekeeping in watches, computers and other electronic equipment.
A crystal of piezoelectric material such as quartz can change shape slightly when a voltage is applied to an electrode on the crystal. Once that voltage is removed, the crystal can return to its original shape and generate a voltage as it is doing so. Discovered in 1880 and called piezoelectricity, this property is critical to the operation of an oscillator. Experiments were carried out with different crystal materials over time and the first quartz oscillator was assembled in the late 1910s. Since the 1920s, quartz crystal oscillators have been frequently used in clocks as well as amateur, commercial and military radio equipment.
When a quartz oscillator is first powered up, the circuit introduces a random noise signal to the crystal. Some of that noise will always be at the crystal's resonant frequency, which causes the crystal to oscillate. The voltage produced by the crystal as it changes shape is amplified by the quartz oscillator circuit and fed back to the crystal resonator. As this process repeats, the signals within the crystal's limited frequency band will become stronger while other frequencies will be filtered out. Once this "warm-up" period is complete, the oscillator will be operating precisely at its designed frequency.
The shape, size and cut of a quartz crystal determine how quickly it expands and contracts. A quartz oscillator can operate at this rate, called its resonant frequency. It can also operate at an overtone frequency, which is a multiple of the resonant frequency. While quartz crystals occur naturally in the environment, a very large number are manufactured in order to increase the yield and supply of physically usable crystals.
The frequency output by a quartz oscillator can be affected by a variety of external factors including temperature and even sudden acceleration. Radiation will also affect frequency, whether it is from cosmic rays in a spacecraft, X-rays or a pulse of ionizing radiation. Some of these factors can be compensated for with extra circuitry which monitors conditions and adjusts the oscillator output accordingly. Some quartz oscillators include a precisely-controlled oven with the crystal inside to compensate for temperature changes. Quartz crystals can also be "swept" or hardened against radiation by baking them in a specialized atmosphere and electrical field.