beat frequency oscillator (BFO)

A beat frequency oscillator (BFO) is an electronic RF circuit for generating variable audio frequencies.

The generation of variable audio frequencies is difficult to realize with analog sine wave generators due to the enormous frequency ratio of 1:1,000 between highest and lowest frequency - namely 20 kHz to 20 Hz. For this reason, the beating principle was already being used in the 1960s. Here, two high-frequency sinusoidal signals are generated, one sinusoidal signal having a fixed frequency, the other a variable frequency that differs only slightly from the fixed frequency. For example, the fixed frequency could be 100 kHz and the variable frequency could be variable between 101 kHz and 110 kHz. If one mixes both frequencies according to the superheterodyne principle, then the resulting difference frequency can be varied between 1 kHz and 10 kHz, and with a constant amplitude.

The beat buzzer or beat superheterodyne was a tone generator in the 1960s and 1970s that could be used to cover a wide range of audio frequencies.