By: Preethi Ramkumar
Meaning of XO - "Crystal Oscillator", A crystal oscillator is an electronic device that uses the mechanical resonance of a physical crystal of piezo-electric 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 rubidium or 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 can be embedded in integrated circuits
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A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. This means that a crystal is very predictable, because of its shape.
Almost any physical object could be used in a similar way to the crystal, with appropriate transducers, since all objects have natural resonant frequencies of vibration. These frequencies depend on size, shape, material, molecular structure, etc. They can be approximately calculated. However, you probably don't want to, because calculating this value for your kitchen table can become extremely complex. That's why crystals are used. Their repeating pattern forms an ideal base for calculating their natural frequency.
Many materials 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.
When the crystal oscillator circuit notices this, it will adjust its frequency. When the frequency gets closer to the natural frequency of the crystal, the amplitude of the crystals movement will suddenly get a lot bigger. For the crystal oscillator circuit this means that there is suddenly almost no resistance at all, and that the current frequency is very close to the natural frequency of the crystal. This is called resonance.
This is usually used by microcontrollers, which can be optimized for system on chip-like situations. Because of the use of crystals, the generated clock signal can be extremely accurate, making it useful for timing systems at high speed.
The crystal oscillator circuit sustains oscillation by taking a voltage signal from the resonator, amplifying it, and feeding it back to the resonator. The rate of expansion and contraction is the resonance frequency, and is determined by the cut and size of the crystal
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An abbreviation for crystal is "X" and 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).
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 Χ 107/f, where f is the resonance frequency in MHz.
Environmental changes of temperature, humidity, pressure, and vibration can change the resonance frequency of a quartz crystal, but there are several designs that reduce these environmental effects. These include the TCXO, MCXO, and OCXO . 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.
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 oscillator that uses a quartz crystal to generate a frequency. Such devices generally output a fixed frequency, but some can be controlled by a tuning voltage over a small range. Contrast with VCO.
[Answers.com]
A crystal (or SAW) oscillator outputs a precise fixed-frequency clock signal at a specified nominal value, used for data and processor timing." [Pericom corporation
]
XO - Crystal Oscillator
The device does not have temperature control or temperature compensation. Frequency - temperature characteristics depend on the crystal units. It is also called SPXO Simple Packaged Crystal Oscillator or Clock.
[Fortiming Corporation]
Crystal oscillator often used for computer timing. An oscillator in which the frequency is controlled by a piezoelectric crystal.
[Crystal and frequency Control]
