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
long wave and
medium wave broadcasting stations is derived from an
atomic clock and can be therefore used as frequency standard. ==See also==