Beat frequency oscillator

BFOs are also used to demodulate single-sideband (SSB) signals, making them intelligible, by essentially restoring the carrier that was suppressed at the transmitter.

In continuous wave (CW) radio transmission, also called radiotelegraphy, or wireless telegraphy (W/T) 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 resulting damped waves (ITU Class B) could be received on a basic crystal set employing a diode detector and an ear phone as a spark rate tone.

It was only with the introduction of tube transmitters that were able to create streams of continuous radio frequency carrier, that a BFO was required.

The alternative was to modulate the carrier with an audio tone around 800 Hz and key the modulated carrier to permit use of the basic diode detector in the receiver, a method used for medium frequency (MF) marine communications up to 2000 (emission type A2A).

Radio transmission using tubes started to replace spark transmitters at sea from 1920 onwards but were not eliminated before 1950[citation needed].

Thus the BFO makes the "dots" and "dashes" of the Morse code signal audible, sounding like different length "beeps" in the speaker.

There is also usually a knob on the front panel to adjust the frequency of the BFO, to change the tone over a small range to suit the operator's preference.

To make them audible, the frequency needs to be shifted into the audio range, for instance faudio = 1000 Hz.