Beatfrekvensoscillator

Beatfrekvensoscillator, often referred to in English as beat frequency oscillator (BFO), is a local oscillator used in some radio receivers to generate an audible reference signal for demodulating certain modulation modes. The oscillator operates at or near the receiver’s intermediate frequency and is mixed with the IF signal in the detector, producing an audio-frequency beat that can be heard as sound.

In practice, the BFO is used to recover information from signals that lack conventional amplitude modulation, notably continuous-wave (CW) Morse code and single-sideband (SSB). For CW, the transmitted signal has a carrier with on‑off keying and little or no envelope; the BFO provides a stable carrier nearby, and the detector yields an audible tone whose pitch corresponds to the beat between the BFO and the received carrier. In SSB, the BFO reintroduces a carrier at the appropriate frequency so that a product detector can reproduce the original audio. The stability and tuneability of the BFO are important, since drift or incorrect offset can make the recovered audio sound warped or unintelligible.

Technically, BFOs are typically simple LC oscillators built with transistors or tubes and paired with the receiver’s detector circuitry. They are commonly associated with receivers using fixed intermediate frequencies, such as around a few hundred kilohertz, though exact implementations vary. In modern equipment, digital signal processing can emulate the same beat-note demodulation concept, but the term BFO remains a traditional descriptor for the role the oscillator plays in CW and SSB demodulation.

A related use of the term exists in metal detectors, where a beat frequency oscillator compares two oscillators (one reference, one in the search coil) to produce an audible tone, with the frequency difference shifting when metal is detected.