A miniature 4 MHz quartz crystal enclosed in a hermetically sealed HC-49/US package, used as the resonator in a crystal oscillator. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in A hermetic seal is a seal which for practical purposes is considered airtight

A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. An electronic circuit is a closed path formed by the interconnection of Electronic components through which an Electric current can flow In Physics, resonance is the tendency of a system to Oscillate at maximum Amplitude at certain frequencies, known as the system's In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating Piezoelectricity is the ability of some materials (notably Crystals and certain Ceramics including bone to generate an Electric potential in response to Frequency is a measure of the number of occurrences of a repeating event per unit Time. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters/receivers. A quartz clock is a Clock that uses an Electronic oscillator that is regulated by a Quartz crystal to keep time In Electronics and especially synchronous Digital circuits a clock signal is a signal used to coordinate the actions of two or more circuits A digital system uses discrete (discontinuous values usually but not always Symbolized Numerically (hence called "digital" to represent information for Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside For biologic transmitters see Transmitter substance. A transmitter is an electronic device which usually with the aid of an antenna This article is about a radio receiver for other uses see Radio (disambiguation.

History

Inside construction of a modern high performance HC-49 package quartz crystal

Piezoelectricity was discovered by Jacques and Pierre Curie in 1880. Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Paul-Jacques Curie ( October 29, 1856 &ndash 1941 was a French Physicist and professor at the University of Montpellier. Pierre Curie (15 May 1859 &ndash 19 April 1906 was a French physicist, a pioneer in Crystallography, Magnetism, Piezoelectricity Paul Langevin first investigated quartz resonators for use in sonar during World War I. Paul Langevin (23 January 1872 &ndash 19 December 1946 was a prominent French Physicist who developed Langevin dynamics and the Sonar (which started as an Acronym for sound navigation and ranging) is a technique that uses Sound propagation (usually underwater to navigate The first crystal controlled oscillator, using a crystal of Rochelle salt, was built in 1917 and patented^[1] in 1918 by Alexander M. An electronic oscillator is an Electronic circuit that produces a repetitive electronic signal often a Sine wave or a Square wave. Potassium sodium tartrate is a Double salt first prepared (in about 1675) by an apothecary, Pierre Seignette, of La Rochelle, Nicholson at Bell Telephone Laboratories, although his priority was disputed by Walter Guyton Cady. Bell Laboratories (also known as Bell Labs and formerly known as AT&T Bell Laboratories and Bell Telephone Laboratories) is the Research organization Dr Walter Guyton Cady ( December 10, 1874 – December 9, 1974) was a noted American physicist and Electrical engineer. ^[2] Cady built the first quartz crystal oscillator in 1921. ^[3] Other early innovators in quartz crystal oscillators include G. W. Pierce and Louis Essen. George Washington Pierce ( January 11, 1872 - August 25, 1956) was an American Physicist. Louis Essen ( September 6, 1908 &ndash August 24, 1997) was an English Physicist whose most notable achievements were in

Operation

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. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge

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. In Physics, resonance is the tendency of a system to Oscillate at maximum Amplitude at certain frequencies, known as the system's For example, steel is very elastic and has a high speed of sound. Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0 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. A material is said to be elastic if it deforms under stress (e 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 a tuning fork. A tuning fork is an acoustic Resonator in the form of a two-pronged Fork with the tines formed from a U-shaped bar of elastic For applications not needing very precise timing, a low-cost ceramic resonator is often used in place of a quartz crystal. Description A ceramic resonator is an Electronic component that can produce Oscillation at a specific Frequency when combined with other

When a crystal of quartz is properly cut and mounted, it can be made to distort in an electric field by applying a voltage to an electrode near or on the crystal. Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e This property is known as piezoelectricity. Piezoelectricity is the ability of some materials (notably Crystals and certain Ceramics including bone to generate an Electric potential in response to 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 an inductor, capacitor and resistor, with a precise resonant frequency. An inductor is a passive electrical component designed to provide Inductance in a circuit A capacitor is a passive electrical component that can store Energy in the Electric field between a pair of conductors |- align = "center"| |width = "25"| | |- align = "center"| || Potentiometer |- align = "center"| | | |- align = "center"| Resistor| | (See RLC circuit. An RLC circuit (also known as a Resonant circuit tuned circuit or LCR circuit is an Electrical circuit consisting of a Resistor (R an )

Quartz has the further advantage that its elastic constants and its size change in such a way that the frequency dependence on temperature can be very low. The specific characteristics will depend on the mode of vibration and the angle at which the quartz is cut (relative to its crystallographic axes)¹ 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 a crystal oven, and can also be mounted on shock absorbers to prevent perturbation by external mechanical vibrations. A crystal oven is a temperature-controlled chamber used to maintain the quartz crystal in electronic Crystal oscillators at a constant temperature in order to prevent changes

Quartz timing crystals are manufactured for frequencies from a few tens of kilohertz to tens of megahertz. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. More than two billion (2×10⁹) crystals are manufactured annually. Most are small devices for consumer devices such as wristwatches, clocks, radios, computers, and cellphones. A watch is a timepiece that is made to be worn on a person The term now usually refers to a wristwatch, which is worn on the wrist with a strap or Bracelet. Circadian Locomotor Output Cycles Kaput, or Clock is a gene which encodes proteins regulating Circadian rhythm. Radio is the transmission of signals by Modulation of electromagnetic waves with frequencies below those of visible Light. A computer is a Machine that manipulates data according to a list of instructions. Quartz crystals are also found inside test and measurement equipment, such as counters, signal generators, and oscilloscopes. A signal generator, also known variously as a test signal generator, function generator, tone generator, arbitrary waveform generator, digital An oscilloscope (commonly abbreviated to scope or O-scope) is a type of Electronic test equipment that allows signal Voltages to be viewed

Modeling

Electrical model

Electronic symbol for a piezoelectric crystal resonator

Schematic symbol and equivalent circuit for a quartz crystal in an oscillator

A quartz crystal can be modelled as an electrical network with a low impedance (series) and a high impedance (parallel) resonance point spaced closely together. An electronic symbol is a Pictogram used to represent various Electrical and electronic devices (such as Wires batteries, Resistors Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal Alternating current (AC Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal Alternating current (AC Mathematically (using the Laplace transform) the impedance of this network can be written as:

or,

where s is the complex frequency (s = jω), ω_s is the series resonant frequency in radians per second and ω_p is the parallel resonant frequency in radians per second. In Mathematics, the Laplace transform is one of the best known and most widely used Integral transforms It is commonly used to produce an easily soluble algebraic The radian is a unit of plane Angle, equal to 180/ π degrees, or about 57

Adding additional capacitance across a crystal will cause the parallel resonance to shift downward. This can be used to adjust the frequency at which a crystal oscillator oscillates. 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.

Resonance modes

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 between series and parallel resonance, which means that the crystal appears as an inductive reactance in operation. 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 its overtones, presented as being a fundamental, 3rd, 5th, or even 7th overtone crystal. An overtone is a natural resonance or vibration frequency of a system The oscillator electronic circuits usually provides additional LC circuits to select the wanted overtone of a crystal.

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 due to the quartz crystal.

Electrical oscillators

A crystal used in hobby radio control equipment to select frequency. Radio control (often abbreviated to R/C or simply RC) is the use of radio signals to remotely control a device

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. In Physics, resonance is the tendency of a system to Oscillate at maximum Amplitude at certain frequencies, known as the system's When the energy of the generated output frequencies matches the losses in the circuit, an oscillation can be sustained.

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 noise AC signal to it, and purely by chance, a tiny fraction of the noise will be at the resonant frequency of the crystal. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant The crystal will therefore start oscillating in synchrony with that signal. As the oscillator amplifies the signals coming out of the crystal, the signals in the crystal's frequency band will become stronger, eventually dominating the output of the oscillator. Natural resistance in the circuit and in the quartz crystal filter out all the unwanted frequencies. Electronic filters are Electronic circuits which perform Signal processing functions specifically intended to remove unwanted signal components and/or enhance wanted

The output frequency of a quartz oscillator can be either the fundamental resonance or a multiple of the resonance, called an overtone frequency. In Acoustics and Telecommunication, the harmonic of a Wave is a component Frequency of the signal that is an Integer An overtone is a natural resonance or vibration frequency of a system High frequency crystals are often designed to operate at third, fifth, or seventh overtones.

A major reason for the wide use of crystal oscillators is their high Q factor. For other uses of the terms Q and Q factor see Q value. In Physics and Engineering the quality A typical Q for a quartz oscillator ranges from 10⁴ to 10⁶, compared to perhaps 10² for an LC oscillator. For other uses of the terms Q and Q factor see Q value. In Physics and Engineering the quality An LC circuit is a variety of resonant circuit or tuned circuit and consists of an Inductor, represented by the letter L and a Capacitor, represented 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.

One of the most important traits of quartz crystal oscillators is that they can exhibit very low phase noise. Phase noise is the Frequency domain representation of rapid short-term random fluctuations in the phase of a Wave, caused by Time domain instabilities In many oscillators, any spectral energy at the resonant frequency will be amplified by the oscillator, resulting in a collection of tones at different phases. In a crystal oscillator, the crystal mostly vibrates in one axis. Therefore, only one phase is dominant. This property of low phase noise makes them particularly useful in telecommunications where stable signals are needed, and in scientific equipment where very precise time references are needed.

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. An atomic clock is a type of Clock that uses an Atomic resonance Frequency standard as its timekeeping element

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. 58 MHz, 10 MHz, 14. 318 MHz, 20 MHz, 33. 33 MHz, and 40 MHz. The vast popularity of the 3. 58 MHz and 14. 318 MHz crystals is attributed initially to low cost resulting from scale of economy resulting from the popularity of television and the fact that this frequency is involved in synchronizing to the colorburst signal necessary to display color on an NTSC or PAL based television set. Mass production (also called flow production, repetitive flow production, series production, or serial production) is the production of In Composite video, colorburst is a signal used to keep the Chrominance subcarrier synchronized in a Color television signal NTSC ( National Television System Committee) is the Analog television system used in the United States, Canada, Japan, Mexico PAL, short for Phase Alternating Line, is a colour -encoding system used in Broadcast television systems in large parts of the world Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic Using frequency dividers, frequency multipliers and phase locked loop circuits, it is practical to derive a wide range of frequencies from one reference frequency. A frequency divider is an Electronic circuit that takes an input signal with a Frequency, f_{in} and generates an output signal with a frequency A phase-locked loop or phase lock loop (PLL is a Control system that generates a signal that has a fixed relation to the phase of a "reference"

Care must be taken to use only one crystal oscillator source when designing circuits to avoid subtle failure modes of metastability in electronics. Metastability in electronics is the ability of a non-equilibrium electronic state to persist for a long (and theoretically unlimited period of time (see Asynchronous circuit 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.

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.

Commonly used crystal frequencies

Circuit notations and abbreviations

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. ) (See IEEE Std 315-1975, or ANSI Y32. The Institute of Electrical and Electronics Engineers or IEEE (read eye-triple-e) is an international Non-profit, professional organization 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 oscillator
• OCXO — oven-controlled crystal oscillator
• RbXO — rubidium crystal oscillators (RbXO), a crystal oscillator (can be a MCXO) synchronized with a built-in rubidium standard which is run only occasionally to save power
• TCVCXO — temperature-compensated voltage-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

See also

• Oscillator
• Electronic oscillator
• Pierce oscillator
• VFO — variable-frequency oscillator
• Crystal oven
• Clock drift - Clock drift measurements of crystal oscillators can be used to build random number generators. In physics an Analog Temperature Controlled Crystal Oscillator or Analogue Temperature Compensated Crystal Oscillator (ATCXO uses analog sampling techniques microcomputer is a Computer with a Microprocessor as its Central processing unit. A crystal oven is a temperature-controlled chamber used to maintain the quartz crystal in electronic Crystal oscillators at a constant temperature in order to prevent changes A crystal oven is a temperature-controlled chamber used to maintain the quartz crystal in electronic Crystal oscillators at a constant temperature in order to prevent changes Rubidium (ruːˈbɪdiəm /rəˈbɪdiəm/ is a Chemical element with the symbol Rb and Atomic number 37 A rubidium standard is a Frequency standard in which a specified hyperfine Transition of Electrons in Rubidium -87 atoms is used to A voltage-controlled oscillator or VCO is an Electronic oscillator designed to be controlled in Oscillation Frequency by a Voltage Oscillation is the repetitive variation typically in Time, of some measure about a central value (often a point of Equilibrium) or between two or more different states An electronic oscillator is an Electronic circuit that produces a repetitive electronic signal often a Sine wave or a Square wave. The Pierce oscillator is a type of Electronic oscillator circuit particularly well-suited for implementing Crystal oscillator circuits A variable frequency oscillator (VFO in Electronics is a Oscillator with an oscillation frequency that can be electronically changed (hence variable A crystal oven is a temperature-controlled chamber used to maintain the quartz crystal in electronic Crystal oscillators at a constant temperature in order to prevent changes Clock drift refers to several related phenomena where a Clock does not run at the exact right speed compared to another clock A random number generator (often abbreviated as RNG is a computational or physical device designed to generate a sequence of Numbers or symbols that lack any

Further reading

• Virgil E Bottom: Introduction to Quartz Crystal Unit Design, 1982

External links

• Introduction to quartz frequency standards
• Quartz crystal application notes and matching reports

References

1. ^ Nicholson, Alexander M. . Generating and transmitting electric currents. US Patent No. 2212845. Online patent database, US Patent and Trademark Office. , filed April 10, 1918, granted August 27, 1940
2. ^ Bottom, Virgil E. (1981). "A history of the quartz crystal industry in the USA". Proc. 35th Frequency Control Symp. , IEEE.  
3. ^ Marrison, Warren (1948). "The Evolution of the Quartz Crystal Clock". Bell System Technical Journal 27: 510-588. AT&T.