Editing Rubidium standard (section)

==Synopsis==
The Rb standard is the most inexpensive, compact, and widely produced [[atomic clock]], used to control the frequency of [[television station]]s, [[cellular base station|cell phone base stations]], in test equipment, and [[global navigation satellite system]]s like [[Global Positioning System|GPS]]. Commercial rubidium clocks are less accurate than [[Caesium standard|caesium atomic clock]]s, which serve as [[Primary standard|primary frequency standards]], so a rubidium clock is usually used as a [[secondary frequency standard]]. 

Commercial rubidium frequency standards operate by disciplining a [[crystal oscillator]] to the rubidium hyperfine transition of 6.8 GHz ({{val|6834682610.904|u=Hz}}). The intensity 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 [[Hyperfine structure|transition frequency]]. The crystal oscillator is stabilized to the rubidium transition by detecting the light dip while sweeping an [[Radio frequency|RF]] [[frequency synthesizer|synthesizer]] (referenced to the crystal) through the transition frequency.