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The Beat Frequency Oscillator is used to demodulate A1 (CW) transmissions. In these morse code transmissions, only a carrier signal is transmitted. In a normal AM radio, one would only hear a very slight hiss in the rhythm of the morse code signals, or even nothing.

So a RF signal very near to the intermediate frequency is added and the mixed signal is demodulated; CW signals are audible as a high pitch tone; in most high grade sets, the BFO frequency and therefore the pitch of the morse code signal is variable.

The BFO also be activated to demodulate single sideband transmissions, the BFO pitch note has to be regulated, so that the human speech sounds natural.