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. It is the most inexpensive, compact, and widely used type of atomic clock, used to control the frequency of television stations, cell phone base stations, in test equipment, and global navigation satellite systems like GPS. Commercial rubidium clocks are less accurate than cesium atomic clocks which serve as primary frequency standards, so the rubidium clock is a secondary frequency standard. However, rubidium fountains are currently being developed that are even more stable than caesium fountain clocks.

All commercial rubidium frequency standards operate by disciplining a crystal oscillator to the rubidium hyperfine transition of 6 834 682 610.904 324 Hz. The amount of light from a rubidium discharge lamp that reaches a photodetector through a resonance cell will drop by about 0.1% when the rubidium vapor in the resonance cell is exposed to microwave power near the transition frequency. The crystal oscillator is stabilized to the rubidium transition by detecting the light dip while sweeping an RF synthesizer (referenced to the crystal) through the transition frequency.