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A crystal oscillator is an electronic device that uses the mechanical resonance of a physical crystal of piezoelectric material to create an electrical signal with a very precise frequency. It is an especially accurate form of an electronic oscillator. The crystal is usually made of quartz, but can also be made of ceramic. This frequency is used to keep track of time (as in quartz wristwatches), to provide a stable clock for digital circuits, and to stabilize frequencies for radio transmitters. Crystal oscillators are the most common source of time and frequency signals. The crystal is sometimes called a "timing crystal". They are often used as the timing reference for integrated circuits.
Crystals for timing purposes
Almost any object made of an elastic material could be used like a crystal, with appropriate transducers, since all objects have natural resonant frequencies of vibration. For example, steel is very elastic and has a high speed of sound. It was often used in mechanical filters before quartz. The resonent frequency depends on size, shape, elasticity and the speed of sound in the material.
Any elastic material can be formed into plates that will resonate. However, since quartz can be directly driven by an electric signal (it is piezoelectric), no additional electrical-to-mechanical transducer is required. This saves money.
When a crystal of quartz is properly cut and mounted, it can be made to bend in an electric field, by applying a voltage to an electrode near or on the crystal. This property is known as piezoelectricity. When the field is removed, the quartz will generate an electric field as it returns to its previous shape, and this can generate a voltage. The result is that a quartz crystal behaves like a capacitor with precise charging and discharging rates.
Quartz has the further advantage that it does not change size much as temperature changes. Therefore, the resonant frequency of the plate, which depends on the plate's size, will not change much, either. This means that a quartz clock, filter or oscillator will be accurate as the temperature changes.
More than two billion (2 × 109) quartz oscillators are manufactured annually. Most are small devices built for wristwatches, clocks, and electronic circuits. However, quartz oscillators are also found inside test and measurement equipment, such as counters, signal generators, and oscilloscopes.
Crystals and frequency
The crystal oscillator circuit sustains oscillation by taking a voltage signal from the quartz resonator, amplifying it, and feeding it back to the resonator. The rate of expansion and contraction of the quartz is the resonant frequency, and is determined by the cut and size of the crystal.
A regular timing crystal contains two electrically conductive plates, with a slice or tuning fork of quartz crystal sandwiched between them. The circuitry around the crystal then applies a noisy, pretty random AC signal to it, and purely by chance, a tiny fraction of the noise will be at the resonant frequency of the crystal. The crystal will therefore start oscillating in synchrony with that signal. As the oscillator amplifies the signals coming out of the crystal, the crystal's frequency will become stronger, eventually dominating the output of the oscillator. Natural resistance in the circuit, and the quartz crystal therefore filter out all the unwanted frequencies.
One of the important traits of a quartz crystal oscillator is that it has very low phase noise. In many oscillators, any signal near the correct frequency will be amplified by the oscillator. The result is a mushy combination of many signals that have the right frequency, but are at different timings. In a crystal oscillator, the crystal mostly moves in just one direction at a time. Therefore, only one phase is very powerful compared to any others.
A quartz oscillator is often used by microcontrollers, which can be optimized for system on chips. A quartz oscillator's accuracy and low phase noise make it useful as a timing reference for high speed digital logic.
The output frequency of a quartz oscillator is either the fundamental resonance or a multiple of the resonance, called an overtone frequency.
A typical Q for a quartz oscillator ranges from 104 to 106. The maximum Q for a high stability quartz oscillator can be estimated as Q = 1.6 × 107/f, where f is the resonance frequency in MHz.
Environmental changes of temperature, humidity, pressure, and vibration can change the resonant frequency of a quartz crystal, but there are several designs that reduce these environmental effects. These include the TCXO, MCXO, and OCXO (defined below). These designs (particularly the OCXO) often produce devices with excellent short-term stability. The limitations in short-term stability are due mainly to noise from electronic components in the oscillator circuits. Long term stability is limited by aging of the crystal.
Due to aging and environmental factors such as temperature and vibration, it is hard to keep even the best quartz oscillators within 10-10 of their nominal frequency without constant adjustment. For this reason, atomic oscillators are used for applications that require better long-term stability and accuracy.
An abbreviation used for a crystal is "X" (or "XTAL"), and for a crystal oscillator is "XO". These abbreviations are used in electronic schematics and in radio specifications.
Types of crystal oscillators include voltage-controlled crystal oscillators (VCXO), temperature-compensated crystal oscillators (TCXO) , oven-controlled crystal oscillators (OCXO), temperature-compensated-voltage controlled crystal oscillators (TCVCXO), oven-controlled voltage-controlled crystal oscillators (OCVCXO), microcomputer-compensated crystal oscillators (MCXO), and rubidium crystal oscillators (RbXO).
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