Editing Beat frequency oscillator (section)

== Overview ==
In [[continuous wave]] (CW) radio transmission, also called [[radiotelegraphy]], or [[Wireless telegraphy|wireless telegraphy (W/T)]] or [[on-off keying]] and designated by the [[International Telecommunication Union]] as [[Types of radio emissions|emission type A1A]], information is transmitted by pulses of [[modulation|unmodulated]] radio [[carrier wave]] which spell out text messages in [[Morse code]]. The different length pulses of carrier, called "dots" and "dashes" or "dits" and "dahs", are produced by the operator switching the [[transmitter]] on and off rapidly using a [[switch]] called a [[telegraph key]]. The first type of transmission was generated using a spark, since the spark fired at around 1000 times a second (when the telegraph key was pressed). 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&nbsp;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|date=May 2021}}.

Since the pulses of carrier have no audio [[modulation]], a CW signal received by an AM [[radio receiver]] simply sounds like "clicks". Sometimes, when the carrier pulses are strong enough to block out the normal static atmospheric "hiss" in the receiver, CW signals could be heard without a BFO as "pulses" of silence. However this was not a reliable method of reception. In order to make the carrier pulses audible in the receiver, a beat frequency oscillator is used. The BFO is a [[radio frequency]] [[electronic oscillator]] that generates a constant sine wave at a frequency ''f''<sub>BFO</sub> that is offset from the [[intermediate frequency]] ''f''<sub>IF</sub> of the receiver. This signal is mixed with the IF before the receiver's second detector ([[demodulator]]). In the detector the two frequencies add and subtract, and a [[beat frequency]] ([[heterodyne]]) in the [[audio signal|audio]] range results at the difference between them:  ''f''<sub>audio</sub> = |''f''<sub>IF</sub> - ''f''<sub>BFO</sub>| which sounds like a tone in the receiver's speaker. During the pulses of carrier, the beat frequency is generated, while between the pulses there is no carrier so no tone is produced. Thus the BFO makes the "dots" and "dashes" of the Morse code signal audible, sounding like different length "beeps" in the speaker. A listener who knows Morse code can decode this signal to get the text message.

The first BFOs, used in early [[tuned radio frequency receiver|tuned radio frequency]] (TRF) receivers in the 1910s-1920s, beat with the carrier frequency of the station. Each time the radio was tuned to a different station frequency, the BFO frequency had to be changed also, so the BFO oscillator had to be tunable across the entire frequency band covered by the receiver.

Since in a [[superheterodyne]] receiver the different frequencies of the different stations  are all translated to the same [[intermediate frequency]] (IF) by the [[frequency mixer|mixer]], modern BFOs which beat with the IF need only have a constant frequency. There may be a switch to turn off the BFO when it is not needed, when receiving other types of signals, such as AM or [[Frequency modulation|FM]]. 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.