Crystal oven

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 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 abbreviation 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.

Miniature crystal oven used to stabilize the frequency of a vacuum-tube mobile radio transmitter.
An OCXO inside an HP digital frequency counter.

Description

Quartz crystals are widely used in electronic oscillators to precisely control the frequency produced. The frequency at which a quartz crystal resonator 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.

PCB-mounted OCXO from 2016.

The oven is a thermally-insulated enclosure containing the crystal and one or more electrical heating elements. 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 circuit 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 to reach its operating temperature.[1] During this warm-up period, the frequency will not have the full rated stability.

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 (Stress-Compensated) crystals are used. The SC-cut has a wider temperature range over which near-zero temperature coefficient is achieved and thus reduces warmup time.[2] Power transistors are usually used for the heaters instead of resistance heating elements. Their power output is proportional to the current, rather than the square of the current, which linearizes the gain of the control loop.[2]

A common temperature for a crystal oven is 75 °C.[3] but may vary between 30 – 80 °C depending on setup.[4]

Most standard commercial crystals are specified to an environmental temperature of 0 – 70 °C, industrial versions are usually specified to -40 – +85 °C.[5]

Accuracy
Some of the earliest crystal ovens. These precision 100 kHz oven controlled crystal oscillators at the US Bureau of Standards (now NIST) served as the frequency standard for the United States in 1929.

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 watches. 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 1×10−12 over a few seconds, while the long term stability is limited to around 1×10−8 (10 ppb) per year by aging of the crystal.[1] Achieving better accuracy 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 ~30 ns 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.

Comparison with other frequency standards
Oscillator Type* Accuracy** Aging / 10 year Radiation per RAD Power Weight (g)
Crystal oscillator (XO)[6] 10−5 to 10−4 10...20 PPM -2 × 10−12 20 µW 20
Temperature compensated crystal oscillator (TCXO)[6] 10−6 2...5 PPM -2 × 10−12 100 µW 50
Microcomputer compensated crystal oscillator (MCXO)[6] 10−8 to 10−7 1...3 PPM -2 × 10−12 200 µW 100
Oven controlled crystal oscillator (OCXO)[6]
- 5...10 MHz
- 15...100 MHz		 2 × 10−8
5 × 10−7		 2 × 10−8 to 2 × 10−7
2 × 10−6 to 11 × 10−9		 -2 × 10−12 1...3 W 200...500
Rubidium atomic frequency standard (RbXO)[6] 10−9 5 × 10−10 to 5 × 10−9 2 × 10−13 6...12 W 1500...2500
Caesium atomic frequency standard[6] 10−12 to 10−11 10−12 to 10−11 2 × 10−14 25...40 W 10000...20000
Global Positioning System (GPS) 4 × 10−8 to 10−11
[7][8]		  10−13 4 W 340
Radio time signal (DCF77) 4 × 10−13[9] 4.6 W[10] 87[11]

* Sizes range from <5 cm3 for clock oscillators to >30 liters for Cs standards. Costs range from <5 US$ for clock oscillators to >40 000 US$ for Cs standards.

** Including the effects of military environments and one year of aging.

References
  1. "OCXO". Glossary. Time and Frequency Division, NIST. 2008. Archived from the original on 2008-09-15. Retrieved 2008-08-07.
  2. Marvin E., Frerking (1996). "Fifty years of progress in quartz crystal frequency standards". Proc. 1996 IEEE Frequency Control Symposium. Institute of Electrical and Electronic Engineers. pp. 33–46. Archived from the original on 2009-05-12. Retrieved 2009-03-31.
  3. "Temperature Controller for Crystal Oven". freecircuitdiagram.com. Free Circuit Diagram. Retrieved 2009-11-17.
  4. "EKSMA OPTICS - manufacturer of laser components - Oven for Nonlinear Crystals TK7". eksmaoptics.com. Archived from the original on 2012-06-18. Retrieved 2009-11-17.
  5. "IQXO-350, -350I Commercial Oscillator" (PDF). surplectronics.com. Retrieved 2009-11-18.
  6. "Tutorial Precision Frequency Generation Utilizing OCXO and Rubidium Atomic Standards with Applications for Commercial, Space, Military, and Challenging Environments IEEE Long Island Chapter March 18, 2004" (PDF). ieee.li. Retrieved 2009-11-16.
  7. "Time and Frequency - Precisely the Way You Need It" (PDF). spectruminstruments.net. Retrieved 2009-11-18.
  8. "GPS Time and Frequency Reference Receiver" (PDF). leapsecond.com. Retrieved 2009-11-18.
  9. "URSI/IEEE XXIX Convention on Radio Science, Espoo, Finland, November 1-2, 2004" (PDF). vtt.fi. Retrieved 2009-11-18.
  10. "Serial DCF77 radio clock Meinberg C51". meinberg.de. Retrieved 2009-11-18.
  11. "ETH - IfE-Wearable Computing - Miniature pocket-worn motion sensor with DCF77 clock". wearable.ethz.ch. Archived from the original on 2011-07-06. Retrieved 2009-11-18.
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