Beat frequency oscillator

In a radio receiver, a beat frequency oscillator or BFO, is a dedicated oscillator used to create an audio frequency signal from Morse code radiotelegraphy (CW) transmissions to make them audible. The signal from the BFO is mixed with the intermediate frequency signal to create a heterodyne or beat frequency which is heard as a tone in the speaker. BFOs are also used to demodulate single-sideband (SSB) signals, making them intelligible, by essentially replacing the suppressed carrier. BFOs are sometimes included in communications receivers designed for short wave listeners and especially for amateur radio stations, which often receive CW and SSB signals.

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.

In continuous wave (CW) radio transmission, also called radiotelegraphy or on-off keying and designated by the International Telecommunication Union as emission type A1A, information is transmitted by pulses of unmodulated radio carrier wave which spell out text messages in Morse code. The different length pulses of carrier, called "dots" and "dashes", are produced by the operator switching the transmitter on and off rapidly using a switch called a telegraph key. This was the first type of radio transmission, and during the early 20th century was widely used for both private person-to-person messages and commercial telegram traffic. With the rise of other types of modulation its use has declined, and CW is now only used for personal hobbyist messages by radio amateurs and is becoming obsolete.

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