Rubidium Oscillators
Rubidium oscillators are the lowest priced members of the atomic oscillator family. They operate at
6,834,682,608 Hz, the resonance frequency of the rubidium atom (87Rb), and use the rubidium frequency
to control the frequency of a quartz oscillator. A microwave signal derived from the crystal oscillator is
applied to the 87Rb 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 (Figure 10.11).
Rubidium oscillators continue to get smaller and less expensive, and offer perhaps the best price-toperformance
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.
The Q of a rubidium oscillator is about 107. 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.
Stability (σy(τ), at τ = 1 s) is typically 1 × 10−11, and about 1 × 10−12 at 1 day. The frequency offset of a
rubidium oscillator ranges from 5 × 10−10 to 5 × 10−12 after a warm-up period of a few minutes or hours,
so they meet the accuracy requirements of most applications without adjustment.

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