Editing Beat frequency oscillator (section)

{{Short description|Oscillator used in radio receivers}}
[[File:Beat frequency oscillator board.jpg|thumb|right|Add-on 455 kHz homemade BFO board]]
In a [[radio receiver]], a '''beat frequency oscillator''' or '''BFO''' is a dedicated [[electronic oscillator|oscillator]] used to create an audio frequency signal from [[Morse code]] [[radiotelegraphy]] ([[Continuous wave|CW]]) transmissions to make them audible. The signal from the BFO is mixed with the received signal to create a [[heterodyne]] or [[Beat (acoustics)|beat]] frequency which is heard as a tone in the speaker. BFOs are also used to demodulate [[single-sideband modulation|single-sideband]] (SSB) signals, making them intelligible, by essentially restoring the [[carrier wave|carrier]] that was suppressed at the transmitter. BFOs are sometimes included in [[communications receiver]]s designed for [[short wave]] listeners; they are almost always found in communication receivers for [[amateur radio]], which often receive CW and SSB signals.<ref name=ARRL>Larry Wolfgang, Charles Hutchinson (ed), ''The ARRL Handbook for Radio Amateurs Sixty Eighth Edition'', ARRL, {{ISBN|978-0872591684}}-9,  pages 12-29,12-30</ref>

The beat frequency oscillator was invented in 1901 by Canadian engineer [[Reginald Fessenden]]. What he called the "heterodyne" receiver was the first application of the [[heterodyne]] principle.