Editing Beat frequency oscillator

{{Short description|Oscillator used in radio receivers}}
[[File:Beat frequency oscillator board.jpg|thumb|right|Add-on 455 kHz homemade BFO board]]
In a [[radio receiver]], a '''beat frequency oscillator''' or '''BFO''' is a dedicated [[electronic oscillator|oscillator]] used to create an audio frequency signal from [[Morse code]] [[radiotelegraphy]] ([[Continuous wave|CW]]) transmissions to make them audible. The signal from the BFO is mixed with the received signal to create a [[heterodyne]] or [[Beat (acoustics)|beat]] frequency which is heard as a tone in the speaker. BFOs are also used to demodulate [[single-sideband modulation|single-sideband]] (SSB) signals, making them intelligible, by essentially restoring the [[carrier wave|carrier]] that was suppressed at the transmitter. BFOs are sometimes included in [[communications receiver]]s designed for [[short wave]] listeners; they are almost always found in communication receivers for [[amateur radio]], which often receive CW and SSB signals.<ref name=ARRL>Larry Wolfgang, Charles Hutchinson (ed), ''The ARRL Handbook for Radio Amateurs Sixty Eighth Edition'', ARRL, {{ISBN|978-0872591684}}-9,  pages 12-29,12-30</ref>

The beat frequency oscillator was invented in 1901 by Canadian engineer [[Reginald Fessenden]]. What he called the "heterodyne" receiver was the first application of the [[heterodyne]] principle.

== 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.

==Example==
[[File:D 1944 Rohde und Schwarz STT4032 Front Large.jpg|thumb|upright=1.0|Separate BFO oscillators were manufactured for receivers that didn't have them; a Rohde und Schwarz STI4032 from 1944.]]
[[Image:Goldschmidt tone wheel.jpg|thumb|upright=1.5|One of the first crude examples of a BFO, the Goldschmidt [[tone wheel]]. Before vacuum tube [[electronic oscillator|oscillator]]s were invented, the first CW receivers used a wheel with electrical contacts around its rim, spun at a high speed by a motor, to interrupt a current to create a radio frequency signal to beat with the incoming radio signal.  This example, at the Tuckerton transatlantic receiving station in New Jersey in 1917, created a 40 kHz signal.]]

A receiver is tuned to a [[Morse code]] signal, and the receiver's [[intermediate frequency]] ('''IF''') is ''f''<sub>IF</sub> = 45000&nbsp;Hz.  That means the dits and dahs have become pulses of a 45000&nbsp;Hz signal, which is inaudible.

To make them audible, the frequency needs to be shifted into the audio range, for instance ''f''<sub>audio</sub> = 1000&nbsp;Hz.  To achieve that, the desired BFO frequency is ''f''<sub>BFO</sub> = 44000 or 46000 Hz. 
 
When the signal at frequency ''f''<sub>IF</sub> is mixed with the BFO frequency in the [[detector (radio)|detector]] stage of the receiver, this creates two other frequencies or [[heterodyne]]s:  |''f''<sub>IF</sub> &minus; ''f''<sub>BFO</sub>|, and |''f''<sub>IF</sub> + ''f''<sub>BFO</sub>|.    The ''difference frequency'', ''f''<sub>audio</sub> = |''f''<sub>IF</sub> &minus; ''f''<sub>BFO</sub>| = 1000 Hz, is also known as the [[beat frequency]].

The other, the ''sum frequency'', ''(F<sub>if</sub> + F<sub>bfo</sub>)'' = 89000 or 91000&nbsp;Hz, is unneeded. It can be removed by a [[lowpass filter]], such as the radio's speaker, which cannot vibrate at such a high frequency.

''f''<sub>BFO</sub> = 44000 or 46000&nbsp;Hz produces the desired 1000&nbsp;Hz beat frequency and either could be used.

By varying the BFO frequency around 44000 (or 46000) Hz, the listener can vary the output audio frequency; this is useful to correct for small differences between the tuning of the transmitter and the receiver, particularly useful when tuning in [[single sideband]] (SSB) voice. The waveform produced by the BFO ''beats'' against the IF signal in the mixer stage of the receiver. Any drift of the local oscillator or the beat-frequency oscillator will affect the pitch of the received audio, so stable oscillators are used.<ref>Paul Horowitz, Winfield Hill "The Art of Electronics 2nd Ed." Cambridge University Press 1989 {{ISBN|0-521-37095-7}}, page 898</ref>

For single sideband reception,  the BFO frequency is adjusted above or below the receiver intermediate frequency, depending on which sideband is used.<ref name=ARRL/>

==Other uses==

Another form of beat-frequency oscillator is used as an adjustable audio frequency signal generator. The signal from a stable crystal-controlled oscillator is mixed with the signal from a tuneable oscillator; the difference in the audio range is amplified and sent as the output of the signal generator.  By using crystal and adjustable frequencies higher than the audio frequency desired, a wide tuning range can be obtained for a small adjustment in the variable oscillator.<ref>E. G. Lapham, ''An Improved Audio Frequency Generator RP367'', ''Bureau of Standards Journal of Research Vol 7'', United States National Bureau of Standards, 1932 page 691 ff</ref> Although the beat-frequency oscillator can produce an output with low distortion, the two oscillators must be very stable to maintain a constant output frequency.<ref>Frank Spitzer, Barry Howarth, ''Principles of Modern Instrumentation'', Holt, Rinehart and Winston, 1972, {{ISBN|0-03-080208-3}}, page 98</ref>

==References==
<references/>

==Further reading==
* [https://web.archive.org/web/20090511181756/http://www.tpub.com/content/neets/14189/css/14189_57.htm "Radiotelephone"], ''NEETS, Module 17--Radio-Frequency Communication Principles''. Integrated Publishing, Electrical Engineering Training Series.
* [http://www.arrl.org/voice-modes "Voice Modes"], [http://www.arrl.org/ ARRL].

[[Category:Communication circuits]]
[[Category:Electronic oscillators]]
[[Category:Amateur radio]]
[[Category:Electronic design]]
[[Category:Radio electronics]]