Crystal oven
Encyclopedia
A crystal oven is a temperature-controlled chamber used to maintain the quartz crystal in electronic crystal oscillator
s at a constant temperature, in order to prevent changes in the frequency
due to variations in ambient temperature. An oscillator of this type is known as an Oven-Controlled Crystal Oscillator (OCXO, where "XO" is an old acronym for "crystal oscillator".) This type of oscillator achieves the highest frequency stability possible with a crystal. They are typically used to control the frequency of radio transmitters, cellular base stations
, military communications equipment, and for precision frequency measurement.
vibrates depends on its physical dimensions. A change in temperature causes the quartz to expand or contract due to thermal expansion
, changing the frequency
of the signal produced by the oscillator. Although quartz has a very low coefficient of thermal expansion, temperature changes are still the major cause of frequency variation in crystal oscillators.
The oven is a thermally-insulated
enclosure containing the crystal and one or more electrical heating element
s. Since other electronic components in the circuit are also vulnerable to temperature drift, usually the entire oscillator circuit is enclosed in the oven. A thermistor
temperature sensor in a closed-loop control
is used to control the power to the heater and ensure that the oven is maintained at the precise temperature desired. Because the oven operates above ambient temperature, the oscillator usually requires a warm-up period after power has been applied. During this warm-up period, the frequency may not be fully stable.
The temperature selected for the oven is that at which the slope of the crystal's frequency vs. temperature curve is zero, further improving stability. AT- or SC-cut crystals are used. The SC-cut has a wider temperature range over which near-zero temperature coefficient is achieved and thus reduces warmup time. Power transistor
s are usually used for the heaters instead of resistance
heating element
s. Their power output is proportional to the current, rather than the square of the current, which linearizes the gain of the control loop.
A common temperature for a crystal oven is . but may vary between depending on setup.
Most standard commercial crystals are specified to an environmental temperature of , industrial versions are usually specified to .
es. However, in return, the oven-controlled oscillator achieves the best frequency stability possible from a crystal. The short term frequency stability of OCXOs is typically 1x10-12 over a few seconds, while the long term stability is limited to around 1x10-8 (10 ppb) per year by aging of the crystal. Achieving better performance requires switching to an atomic frequency standard
, such as a rubidium standard
, caesium standard
, or hydrogen maser
. Another cheaper alternative is to discipline a crystal oscillator with a GPS time signal, creating a GPS Disciplined oscillator (GPSDO). Using a GPS receiver that can generate accurate time signals (down to within of UTC), a GPSDO can maintain oscillation accuracy of 10-13 for extended periods of time.
Crystal ovens are also used in optics. In crystals used for nonlinear optics
, the frequency is also sensitive to temperature and thus they require temperature stabilization, especially as the laser beam heats up the crystal. Additionally fast retuning of the crystal is often employed. For this application, the crystal and the thermistor need to be in very close contact and both must have as low a heat capacity as possible. To avoid breaking the crystal, large temperature variations in short times must be avoided.
Hierarchy of oscillators:
* Sizes range from for clock oscillators to >30 liters for Cs
standards. Costs range from for clock oscillators to for Cs standards.
** Including the effects of military environments and one year of aging.
For comparison if your clock source has an accuracy of 10-7 (0,1 PPM) the clock will drift under one year: 10-7 * (365.25 * 24 * 60 * 60) = 3.2 seconds.
Another example, if you know that your computer clock drifts 120 seconds per day the accurancy is calculated by = -2.9 rounded to -3. Meaning the accurancy is 10-3. One way of increasing the accurancy in a computer environment is to use the Network Time Protocol
(NTP).
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
s at a constant temperature, in order to prevent changes in the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
due to variations in ambient temperature. An oscillator of this type is known as an Oven-Controlled Crystal Oscillator (OCXO, where "XO" is an old acronym for "crystal oscillator".) This type of oscillator achieves the highest frequency stability possible with a crystal. They are typically used to control the frequency of radio transmitters, cellular base stations
Cell site
A cell site is a term used to describe a site where antennas and electronic communications equipment are placed, usually on a radio mast, tower or other high place, to create a cell in a cellular network...
, military communications equipment, and for precision frequency measurement.
Description
The frequency at which a quartz crystal resonatorResonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...
vibrates depends on its physical dimensions. A change in temperature causes the quartz to expand or contract due to thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
, changing the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the signal produced by the oscillator. Although quartz has a very low coefficient of thermal expansion, temperature changes are still the major cause of frequency variation in crystal oscillators.
The oven is a thermally-insulated
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
enclosure containing the crystal and one or more electrical heating element
Heating element
A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element....
s. Since other electronic components in the circuit are also vulnerable to temperature drift, usually the entire oscillator circuit is enclosed in the oven. A thermistor
Thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...
temperature sensor in a closed-loop control
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
is used to control the power to the heater and ensure that the oven is maintained at the precise temperature desired. Because the oven operates above ambient temperature, the oscillator usually requires a warm-up period after power has been applied. During this warm-up period, the frequency may not be fully stable.
The temperature selected for the oven is that at which the slope of the crystal's frequency vs. temperature curve is zero, further improving stability. AT- or SC-cut crystals are used. The SC-cut has a wider temperature range over which near-zero temperature coefficient is achieved and thus reduces warmup time. Power transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s are usually used for the heaters instead of resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
heating element
Heating element
A heating element converts electricity into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element....
s. Their power output is proportional to the current, rather than the square of the current, which linearizes the gain of the control loop.
A common temperature for a crystal oven is . but may vary between depending on setup.
Most standard commercial crystals are specified to an environmental temperature of , industrial versions are usually specified to .
Accuracy
Because of the power required to run the heater, OCXOs require more power than oscillators that run at ambient temperature, and the requirement for the heater, thermal mass, and thermal insulation means that they are physically larger. Therefore they are not used in battery powered or miniature applications, such as watchWatch
A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...
es. However, in return, the oven-controlled oscillator achieves the best frequency stability possible from a crystal. The short term frequency stability of OCXOs is typically 1x10-12 over a few seconds, while the long term stability is limited to around 1x10-8 (10 ppb) per year by aging of the crystal. Achieving better performance requires switching to an atomic frequency standard
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
, such as a rubidium standard
Rubidium standard
A rubidium standard or rubidium atomic clock is a frequency standard in which a specified hyperfine transition of electrons in rubidium-87 atoms is used to control the output frequency...
, caesium standard
Caesium standard
A caesium standard or caesium atomic clock is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms are used to control the output frequency. They are one of the most accurate types of atomic clock...
, or hydrogen maser
Hydrogen maser
A Hydrogen maser, also known as hydrogen frequency standard, is a specific type of maser that uses the intrinsic properties of the hydrogen atom to serve as a precision frequency reference....
. Another cheaper alternative is to discipline a crystal oscillator with a GPS time signal, creating a GPS Disciplined oscillator (GPSDO). Using a GPS receiver that can generate accurate time signals (down to within of UTC), a GPSDO can maintain oscillation accuracy of 10-13 for extended periods of time.
Crystal ovens are also used in optics. In crystals used for nonlinear optics
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
, the frequency is also sensitive to temperature and thus they require temperature stabilization, especially as the laser beam heats up the crystal. Additionally fast retuning of the crystal is often employed. For this application, the crystal and the thermistor need to be in very close contact and both must have as low a heat capacity as possible. To avoid breaking the crystal, large temperature variations in short times must be avoided.
Hierarchy of oscillators:
| Oscillator Type* | Accuracy** | Aging / 10 year | Radiation per RAD Rad (unit) The rad is a unit of absorbed radiation dose. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the malignant mammalian cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by... |
Power | Weight |
|---|---|---|---|---|---|
| Crystal oscillator Crystal oscillator A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency... (XO) |
10-5 to 10-4 | 10-20 PPM Parts-per notation In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement... |
-2 x 10-12 | 20 µW | 20 gram |
| Temperature compensated crystal oscillator Crystal oscillator A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency... (TCXO) |
10-6 | 2-5 PPM | -2 x 10-12 | 100 µW | 50 gram |
| Microcomputer compensated crystal oscillator Crystal oscillator A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency... (MCXO) |
10-8 to 10-7 | 1-3 PPM | -2 x 10-12 | 200 µW | 100 gram |
| Oven controlled crystal oscillator (OCXO) - |
2 x 10-8 5 x 10-7 |
|
-2 x 10-12 | ||
| Small atomic frequency standard Rubidium standard A rubidium standard or rubidium atomic clock is a frequency standard in which a specified hyperfine transition of electrons in rubidium-87 atoms is used to control the output frequency... (Rb Rubidium Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid... , RbXO) |
10-9 | 5 x 10-10 to 5 x 10-9 | 2 x 10-13 | ||
| High Performance atomic standard Caesium standard A caesium standard or caesium atomic clock is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms are used to control the output frequency. They are one of the most accurate types of atomic clock... (Cs Caesium Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature... ) |
10-12 to 10-11 | 2 x 10-14 | |||
| Global Positioning System Global Positioning System The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites... (GPS) |
|
||||
| Radio time signal Time signal A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:... (DCF77 DCF77 DCF77 is a longwave time signal and standard-frequency radio station. Its primary and backup transmitter are located in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. It is operated by Media Broadcast GmbH , on behalf of the Physikalisch-Technische Bundesanstalt, Germany's... ) |
* Sizes range from for clock oscillators to >30 liters for Cs
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
standards. Costs range from for clock oscillators to for Cs standards.
** Including the effects of military environments and one year of aging.
For comparison if your clock source has an accuracy of 10-7 (0,1 PPM) the clock will drift under one year: 10-7 * (365.25 * 24 * 60 * 60) = 3.2 seconds.
Another example, if you know that your computer clock drifts 120 seconds per day the accurancy is calculated by = -2.9 rounded to -3. Meaning the accurancy is 10-3. One way of increasing the accurancy in a computer environment is to use the Network Time Protocol
Network Time Protocol
The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...
(NTP).