Page 358 - The Mechatronics Handbook

FIGURE 17.11 Rubidium oscillator.

or changes in the quartz material or crystal structure. A high quality OCXO might age at a rate of <5 ×
-9
10 per year, while a TCXO might age 100 times faster.
Due to aging and environmental factors such as temperature and vibration, it is hard to keep even the
-10
best quartz oscillators within 1 × 10 of their nominal frequency without constant adjustment. For this
reason, atomic oscillators are used for applications that require better long-term accuracy and stability
[4,13,14].

Rubidium Oscillators

Rubidium oscillators are the lowest priced members of the atomic oscillator family. They operate at
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6,834,682,608 Hz, the resonance frequency of the rubidium atom ( Rb), and use the rubidium frequency
to control the frequency of a quartz oscillator. A microwave signal derived from the crystal oscillator is
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applied to the Rb vapor within a cell, forcing the atoms into a particular energy state. An optical beam
is then pumped into the cell and is absorbed by the atoms as it forces them into a separate energy state.
A photo cell detector measures how much of the beam is absorbed, and its output is used to tune a
quartz oscillator to a frequency that maximizes the amount of light absorption. The quartz oscillator is
then locked to the resonance frequency of rubidium, and standard frequencies are derived from the
quartz oscillator and provided as outputs (Fig. 17.11).
Rubidium oscillators continue to get smaller and less expensive, and offer perhaps the best price-to-
performance ratio of any oscillator. Their long-term stability is much better than that of a quartz oscillator
and they are also smaller, more reliable, and less expensive than cesium oscillators.
7
The Q of a rubidium oscillator is about 10 . The shifts in the resonance frequency are due mainly to
collisions of the rubidium atoms with other gas molecules. These shifts limit the long-term stability.
-12
-11
Stability (σ y (τ), at τ = 1 s) is typically 1 × 10 , and about 1 × 10 at 1 day. The frequency offset of a
-12
-10
rubidium oscillator ranges from 5 × 10 to 5 × 10 after a warm-up period of a few minutes or hours,
so they meet the accuracy requirements of most applications without adjustment.
Cesium Oscillators

Cesium oscillators are primary frequency standards since the SI second is defined from the resonance
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frequency of the cesium atom ( Cs), which is 9,192,631,770 Hz. A properly working cesium oscillator
should be close to its nominal frequency without adjustment, and there should be no change in frequency
due to aging.
Commercially available oscillators use cesium beam technology. Inside a cesium oscillator, Cs atoms
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are heated to a gas in an oven. Atoms from the gas leave the oven in a high-velocity beam that travels
through a vacuum tube toward a pair of magnets. The magnets serve as a gate that allows only atoms of
a particular magnetic energy state to pass into a microwave cavity, where they are exposed to a microwave

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