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 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 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 ~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.
Hierarchy of oscillators:
| Oscillator Type* | Accuracy** | Aging / 10 year | Radiation per RAD | Power | Weight |
|---|---|---|---|---|---|
| Crystal oscillator (XO) | 7007100000000000000 10-5 to 10-4 | 7008200000000000000 10-20 PPM | 2997800000000000000 -2 x 10-12 | 7001200000000000000 20 µW | 7001200000000000000 20 gram |
| Temperature compensated crystal oscillator (TCXO) | 7005100000000000000 10-6 | 7007500000000000000 2-5 PPM | 2997800000000000000 -2 x 10-12 | 7002100000000000000 100 µW | 7001500000000000000 50 gram |
| Microcomputer compensated crystal oscillator (MCXO) | 7004100000000000000 10-8 to 10-7 | 7007300000000000000 1-3 PPM | 2997800000000000000 -2 x 10-12 | 7002200000000000000 200 µW | 7002100000000000000 100 gram |
| Oven controlled crystal oscillator (OCXO) - 5 - 10 MHz - 15 to 100 MHz |
7004500000000000000 2 x 10-8 5 x 10-7 |
7006200000000000000 2 x 10-8 to 2 x 10-7 2 x 10-6 to 11 x 10-9 |
2997800000000000000 -2 x 10-12 | 7006200000000000000 1 - 3 W | 7002350000000000000 200 - 500 gram |
| Small atomic frequency standard (Rb, RbXO) | 7002100000000000000 10-9 | 7004500000000000000 5 x 10-10 to 5 x 10-9 | 7001200000000000000 2 x 10-13 | 7006900000000000000 6 - 12 W | 7003200000000000000 1500 - 2500 gram |
| High Performance atomic standard (Cs) | 7000100000000000000 10-12 to 10-11 | 7002100000000000000 10-12 to 10-11 | 7000200000000000000 2 x 10-14 | 7007325000000000000 25 - 40 W | 7004150000000000000 10 000 - 20 000 gram |
| Global Positioning System (GPS) | 7003400000000000000 4 * 10-8 to 10-11 |
7000100000000000000 10-13 | 7006400000000000000 4 W | 7002340000000000000 340 gram | |
| Radio time signal (DCF77) | 7009100000000000000 | 7000400000000000000 4 x 10-13 | 7006460000000000000 4,6 W | 7001870000000000000 87 gram |
* 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.
For comparison if a (1 Hz) 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 log10(120 / (24 * 60 * 60)) = -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).
Read more about this topic: Crystal Oven
Famous quotes containing the word accuracy:
“As for farming, I am convinced that my genius dates from an older era than the agricultural. I would at least strike my spade into the earth with such careless freedom but accuracy as the woodpecker his bill into a tree.”
—Henry David Thoreau (1817–1862)
“U.S. international and security policy ... has as its primary goal the preservation of what we might call “the Fifth Freedom,” understood crudely but with a fair degree of accuracy as the freedom to rob, to exploit and to dominate, to undertake any course of action to ensure that existing privilege is protected and advanced.”
—Noam Chomsky (b. 1928)
“The child who has been taught to make an accurate elevation, plan, and section of a pint pot has had an admirable training in accuracy of eye and hand.”
—Thomas Henry Huxley (1825–95)