Crystal oscillator From Wikipedia

Crystal oscillator
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A crystal oscillator is anelectronic circuit that uses the mechanicalresonance of a vibratingcrystal ofpiezoelectric material to create an electrical signal with a very precisefrequency. This frequency is commonly used to keep track of time (as inquartz wristwatches), to provide a stableclock signal fordigitalintegrated circuits, and to stabilize frequencies forradio transmitters.
Using anamplifier andfeedback, it is an especially accurate form of anelectronic oscillator. The crystal used therein is sometimes called a "timing crystal". Onschematic diagrams a crystal is labeled Y.
Contents
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1 Crystals for timing purposes2 Crystal modelling3 Bode magnitude diagram4 Temperature effects5 Crystals and frequency5.1 Commonly used crystal frequencies
6 Series or parallel resonance7 Spurious frequencies8 Notation9 See also10 External links
[edit] Crystals for timing purposes

A miniature 4MHzquartz crystal enclosed in anhermetically sealed HC-49/US package, used as the resonator in a crystal oscillator.

Inside construction of a modern high performance HC-49 packagequartz crystal
Acrystal is asolid in which the constituentatoms,molecules, orions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions.
Almost any object made of an elastic material could be used like a crystal, with appropriate transducers, since all objects have naturalresonant 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 resonant frequency depends on size, shape,elasticity, and the speed of sound in the material. High-frequency crystals are typically cut in the shape of a simple, rectangular plate. Low-frequency crystals, such as those used in digital watches, are typically cut in the shape of atuning fork. For applications not needing very precise timing, a low-costceramic resonator is often used in place of a quartz crystal.
When a crystal ofquartz is properly cut and mounted, it can be made to bend in an electric field by applying avoltage to anelectrode near or on the crystal. This property is known aspiezoelectricity. 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 circuit composed of aninductor,capacitor andresistor, with a precise resonant frequency. (SeeRLC circuit.)
Quartz has the further advantage that its size changes very little with temperature. Therefore, the resonant frequency of the plate, which depends on its size, will not change much, either. This means that a quartz clock, filter or oscillator will remain accurate. For critical applications the quartz oscillator is mounted in a temperature-controlled container, called acrystal oven, and can also be mounted on shock absorbers to prevent perturbation by external mechanical vibrations.
Quartz timing crystals are manufactured for frequencies from a few tens ofkilohertz to tens ofmegahertz. More than two billion (2×109) crystals are manufactured annually. Most are small devices for consumer devices such aswristwatches,clocks,radios,computers, andcellphones. Quartz crystals are also found inside test and measurement equipment, such as counters,signal generators, andoscilloscopes.
[edit] Crystal modelling
A quartz crystal can be modelled as an electrical network with a lowimpedance (series) and a highimpedance (parallel) resonance point spaced closely together. Mathematically the impedance of this network can be written as:

or,

where s is the complex frequency (s = jω), ωs is the series resonant frequency inradians per second and ωp is the parallel resonant frequency in radians per second.
Adding additional capacitance across a crystal will cause the parallel resonance to shift downward. This can be used to adjust the frequency that a crystal oscillator oscillates at. Crystal manufacturers normally cut and trim their crystals to have a specified resonant frequency with a known ‘load‘ capacitance added to the crystal. For example, a 6 pF 32 kHz crystal has a parallel resonance frequency of 32,768 Hz when a 6.0 pF capacitor is placed across the crystal. Without this capacitance, the resonance frequency is higher than 32,768 Hz.
[edit] Bode magnitude diagram
The presentBode magnitude diagram illustrates the fact that crystal oscillators are extremely selective.

[edit] Temperature effects
A crystal‘s frequency characteristic depends on the shape or ‘cut‘ of the crystal. A tuning fork crystal is usually cut such that its frequency over temperature is a parabolic curve centered around 25 °C. This means that a tuning fork crystal oscillator will resonate close to its target frequency at room temperature, but will slow down when the temperature either increases or decreases from room temperature. A common parabolic coefficient for a 32 kHz tuning fork crystal is −0.04 ppm/°C².

In a real application, this means that a clock built using a regular 32 kHz tuning fork crystal will keep good time at room temperature, lose 2 minutes per year at 10 degrees Celsius above (or below) room temperature and lose 8 minutes per year at 20 degrees Celsius above (or below) room temperature.
[edit] Crystals and frequency

Schematic symbol and equivalent circuit for a quartz crystal in an oscillator
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 theresonant 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. During startup, the circuit around the crystal applies a random noiseAC 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 in the quartz crystalfilter out all the unwanted frequencies.
One of the most important traits of quartz crystal oscillators is that they can exhibit very lowphase noise. In other words, the signal they produce is apure tone. This makes them particularly useful in telecommunications where stable signals are needed, and in scientific equipment where very precise time references are needed.
The output frequency of a quartz oscillator is either the fundamental resonance ora multiple of the resonance, called anovertone frequency.
A typicalQ 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 megahertz.
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 one part in 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.
Although crystals can be fabricated for any desired resonant frequency, within technological limits, in actual practice today engineers design crystal oscillator circuits around relatively few standard frequencies, such as 3.58MHz, 10 MHz, 14.318, 20 MHz, 33.33 Mhz, and 40 MHz. The vast popularity of the 3.58MHz and 14.318MHz crystals is attributed initially to low cost resulting fromscale of economy resulting from the popularity of television and the fact that this frequency is involved in synchronizing to thecolorburst signal necessary to display color on anNTSC orPAL basedtelevision set. Usingfrequency dividers,frequency multipliers andphase locked loop circuits, it is possible to synthesize any desired frequency from the reference frequency.
Care must be taken to use only one crystal oscillator source when designing circuits to avoid subtle failure modes ofmetastability in electronics. If this is not possible, the number of distinct crystal oscillators, PLLs, and their associated clock domains should be rigorously minimized, through techniques such as using a subdivision of an existing clock instead of a new crystal source. Each new distinct crystal source needs to be rigorously justified, since each one introduces new, difficult to debug probabilistic failure modes, due to multiple crystal interactions, into equipment.
[edit] Commonly used crystal frequencies
Frequency (MHz)   Primary uses  
32.768 kHzReal-time clocks, allows binary division to 1 Hz signal (215 x 1 Hz); also often used in low-speed low-power circuits
1.8432UART clock; allows integer division to commonbaud rates
2.4576UART clock; allows integer division to common baud rates up to 38400
3.2768 allows binary division to 100 Hz (32768x 100 Hz, or 215 x 100 Hz)
3.575611PALM colorsubcarrier
3.579545NTSC M color subcarrier; very common and cheap, used in many other applications, eg.DTMF generators
3.582056PALN color subcarrier
3.686400UART clock (2x 1.8432 MHz); allows integer division to common baud rates
4.096000 allows binary division to 1 kHz (212 x 1 kHz)
4.194304Real-time clocks, clearly divides to 1 Hz signal (222 x 1 Hz)
4.433618PALB/D/G/H/I and NTSC M4.43 color subcarrier
4.9152 Used inCDMA systems; divided to 1.2288 MHz baseband frequency as specified by J-STD-008
5.068 used in radio transceivers as anIF source
6.144 digital audio systems -DAT,MiniDisc,sound cards; 128x 48 kHz (27 x 48 kHz). Also allows integer division to common UART baud rates up to 38400.
6.5536 allows binary division to 100 Hz (65536x 100 Hz, or 216 x 100 Hz); used also inred boxes
7.15909 NTSC M color subcarrier (2x 3.579545 MHz)
7.3728UART clock (4x 1.8432 MHz); allows integer division to commonbaud rates
8.86724 PAL B/G/H color subcarrier (2x 4.433618 MHz)
9.216 allows integer division to 1024 kHz and its halves (16 kHz, 32 kHz, 64 kHz...)
9.83040 Used inCDMA systems (2x 4.9152); divided to 1.2288 MHz baseband frequency
10.245 used in radio transceivers; mixes with 10.7 MHz subcarrier yielding 455 kHz signal, a common secondIF forFM radio and first IF forAM radio[1]
10.700 used in radio transceivers as anIF source
11.0592UART clock (6x 1.8432 MHz); allows integer division to common baud rates
11.2896 Used incompact disc digital audio systems andCDROM drives; allows binary division to 44.1 kHz (256x 44.1 kHz), 22.05 kHz, and 11.025 kHz
12.288 digital audio systems -DAT,MiniDisc,sound cards; 256x 48 kHz (28 x 48 kHz). Also allows integer division to common UART baud rates up to 38400.
13.875 used in someteletext circuits; 2x 6.9375 MHz (clock frequency of PAL B teletext; SECAM uses 6.203125 MHz, NTSC M uses 5.727272 MHz, PAL G uses 6.2031 MHz, and PAL I uses 4.4375 MHz clock)
14.3182 NTSC M color subcarrier (4x 3.579545 MHz). Also common onVGA cards.
14.7456UART clock (8x 1.8432 MHz); allows integer division to common baud rates
16.368 Commonly used for down-conversion and sampling inGPS-receivers. Generatesintermediate frequency signal at +4.092 MHz. 16.3676 or 16.367667 MHz are sometimes used to avoid perfect lineup between sampling frequency and GPSspreading code.
16.9344 Used incompact disc digital audio systems andCDROM drives; allows integer division to 44.1 kHz (384x 44.1 kHz), 22.05 kHz, and 11.025 kHz. Also allows integer division to common UART baud rates.
17.734475 PAL B/G/H color subcarrier (4x 4.433618 MHz)
18.432UART clock (10x 1.8432 MHz); allows integer division to common baud rates. Also allows integer division to 48 kHz (384x 48 kHz), 96 kHz, and 192 kHz samplerates used in high-end digital audio.
19.6608 Used inCDMA systems (4x 4.9152); divided to 1.2288 MHz baseband frequency
24.576 digital audio systems -DAT,MiniDisc,sound cards; 512x 48 kHz (29 x 48 kHz)
29.4912UART clock (16x 1.8432 MHz); allows integer division to common baud rates
[edit] Series or parallel resonance
A quartz crystal provides both series and parallel resonance. The series resonance is a few kilohertz lower than the parallel one. Crystals below 30 MHz are generally operated at parallel resonance, which means that the crystal impedance appears infinite. Any additional circuit capacitance will thus pull the frequency down. For a parallel resonance crystal to operate at its specified frequency, the electronic circuit has to provide a total parallel capacitance as specified by the crystal manufacturer.
Crystals above 30 MHz (up to >200 MHz) are generally operated at series resonance where the impedance appears at its minimum and equal to the series resistance. For this reason the series resistance is specified (<100 Ω) instead of the parallel capacitance. For the upper frequencies, the crystals are operated at one of itsovertones, presented as being a fundamental, 3rd, 5th, or even 7th overtone crystal. The oscillator electronic circuits usually provides additional LC circuits to select the wanted overtone of a crystal.
[edit] Spurious frequencies
For crystals operated in series resonance, significant (and temperature-dependent) spurious responses may be experienced. These responses typically appear some tens of kilohertz above the wanted series resonance. Even if the series resistances at the spurious resonances appear higher than the one at wanted frequency, the oscillator may lock at a spurious frequency (at some temperatures). This is generally avoided by using low impedance oscillator circuits to enhance the series resistance differences.
[edit] Notation
On electrical schematic diagrams, crystals are designated with the class letter "Y" (Y1, Y2, etc.) Oscillators, whether they are crystal oscillators or other, are designated with the class letter "G" (G1, G2, etc.) (SeeIEEE Std 315-1975, orANSI Y32.2-1975) On occasion, one may see a crystal designated on a schematic with "X" or "XTAL", or a crystal oscillator with "XO", but these forms are deprecated.
Crystal oscillator types and their abbreviations:
ATCXO —analog temperature controlled crystal oscillator CDXO —calibrated dual crystal oscillator MCXO —microcomputer-compensated crystal oscillator OCVCXO —oven-controlled voltage-controlled crystal oscillatorOCXO — oven-controlled crystal oscillator RbXO —rubidium crystal oscillators (RbXO), a crystal oscillator (can be a MCXO) synchronized with a built-inrubidium standard which is run only occasionally to save power TCVCXO — temperature-compensatedvoltage-controlled crystal oscillator TCXO — temperature-compensated crystal oscillator TSXO — temperature-sensing crystal oscillator, an adaptation of the TCXO VCTCXO — voltage controlled temperature compensated crystal oscillator VCXO — voltage-controlled crystal oscillator DTCXO — digital temperature compensated crystal oscillator
[edit] See also
OscillatorElectronic oscillatorPierce oscillatorVFO — variable-frequency oscillatorCrystal ovenClock drift - Clock drift measurements of crystal oscillators can be used to buildrandom number generators.
[edit] External links
Introduction to quartz frequency standards
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Category:Oscillators