A frequency reference is an instrument used for providing a stable frequency of some kind. There are different sorts of frequency references, acoustic ones such as tuning forks 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 standards, including caesium fountains, and hydrogen masers. Caesium standards are widely recognized as having better long-term stability, whereas hydrogen masers can attain superior short-term performance; therefore, several national 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 and mediumwave broadcasting stations is derived from an atomic clock and can be therefore used as frequency standard.
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