Editing Intermediate frequency (section)

==History==
An intermediate frequency was first used in the superheterodyne radio receiver, invented by American scientist Major [[Edwin Armstrong]] in 1918, during [[World War I]].<ref name="Redford_1996"/><ref name="Wiccanpiper_2004"/> A member of the [[Signal Corps (United States Army)|Signal Corps]], Armstrong was building radio [[direction finding]] equipment to track German military signals at the then-very high frequencies of 500 to 3500&nbsp;kHz. The [[triode vacuum tube]] amplifiers of the day would not amplify stably above 500&nbsp;kHz; however, it was easy to get them to [[Electronic oscillator|oscillate]] above that frequency. Armstrong's solution was to set up an oscillator tube that would create a frequency near the incoming signal and mix it with the incoming signal in a mixer tube, creating a [[heterodyne]] or signal at the lower difference frequency where it could be amplified easily. For example, to pick up a signal at 1500&nbsp;kHz the local oscillator would be tuned to 1450&nbsp;kHz. Mixing the two created an intermediate frequency of 50&nbsp;kHz, which was well within the capability of the tubes. The name ''superheterodyne'' was a contraction of ''supersonic heterodyne'', to distinguish it from receivers in which the heterodyne frequency was low enough to be directly audible, and which were used for receiving [[continuous wave]] (CW) [[Morse code]] transmissions (not speech or music).

After the war, in 1920, Armstrong sold the patent for the superheterodyne to [[Westinghouse Electric (1886)|Westinghouse]], who subsequently sold it to [[RCA]]. The increased complexity of the superheterodyne circuit compared to earlier [[Regenerative receiver|regenerative]] or [[tuned radio frequency receiver]] designs slowed its use, but the advantages of the intermediate frequency for selectivity and static rejection eventually won out; by 1930, most radios sold were 'superhets'. During the development of [[radar]] in [[World War II]], the superheterodyne principle was essential for downconversion of the very high radar frequencies to intermediate frequencies. Since then, the superheterodyne circuit, with its intermediate frequency, has been used in virtually all radio receivers.