Rubidium standard

In telecommunication, a rubidium standard is a frequency standard in which a specified hyperfine[?] transition of electrons in rubidium-87 atoms is used to control the output frequency.

Note: A rubidium standard consists of a gas cell, which has an inherent long-term instability. This instability relegates the rubidium standard to its status as a secondary standard.

See also: cesium standard

Source: from Federal Standard 1037C