Editing Frequency standard (section)

==Frequency reference==
A '''frequency reference''' is an instrument used for providing a stable frequency of some kind. There are different sorts of frequency references, [[acoustics|acoustic]] ones such as [[tuning fork]]s but also electrical ones that emit a signal of a certain frequency (a frequency standard).

Among the most stable frequency references in the world are [[caesium standard]]s (including [[atomic fountain|caesium fountain]]s) and [[hydrogen maser]]s. Caesium standards are widely recognized as having better long-term stability, whereas hydrogen masers can attain superior short-term performance; therefore, several national [[Measurement standards laboratory|standards laboratories]] use ensembles of caesium standards and hydrogen masers in order to combine the best attributes of both.

The carrier of [[time signal]] transmitters, [[Loran-C]] transmitters and of several [[longwave|long wave]] and [[mediumwave|medium wave]] broadcasting stations is derived from an [[atomic clock]] and can be therefore used as frequency standard.