Beat frequency oscillator

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A beat frequency oscillator or BFO in radio telegraphy, is a dedicated oscillator used to create an audio frequency signal from carrier wave transmissions to make them audible, as they are not broadcast as such. The signal from the BFO is then heterodyned with the intermediate frequency signal to create an audio frequency signal.

[edit] Example

A receiver is tuned to a Morse code signal, and the receiver's intermediate frequency (IF) is F_if = 45000 Hz. That means the dots and dashes [also known as dits and dahs] have become pulses of a 45000 Hz tone, which is inaudible. To make them audible, the frequency needs to be shifted into the audio range, for instance F_baseband = 1000 Hz. To do that, the desired frequency shift is F_bfo = 44000 Hz, and the desired BFO output waveform is cos(2π 44000 t). When the signal at frequency F_if is multiplied by that waveform, it shifts the signal to two other frequencies: |F_if − F_bfo| and (F_if + F_bfo). The difference frequency, |F_if − F_bfo| = 1000 Hz, is also known as the beat frequency. The other frequency, (F_if + F_bfo) = 89000 Hz, can then be removed by a lowpass filter, such as an ordinary speaker (which cannot vibrate at such a high frequency) or the human ear (which is not sensitive to frequencies over approximately 20kHz).

[edit] Notes

1. By varying the BFO frequency around 44000 Hz, the listener can vary the output audio frequency to suit his/her preference.
   That is particularly useful for tuning in single sideband voice.
2. The waveform produced by the BFO is sometimes said to beat against the IF signal, thus the name BFO. However, the actual beat frequency is produced by the mixer, not the BFO.
3. F_bfo = 46000 Hz also produces the desired 1000 Hz beat frequency. These techniques are distinguished by the terms high-side and low-side injection. The term superheterodyne applies to either one, because historically, super just stands for "supersonic". However, a neologism has apparently begun to creep into modern usage, such that the prefixes super- and sub- would be associated with high and low injection, respectively.
4. For a radio signal with more bandwidth than Morse code, low-side injection preserves the relative order of the frequency components. High-side injection reverses their order, which is often desirable to counteract a previous reversal in the radio receiver.