![Beat frequency oscillator board](https://upload.wikimedia.org/wikipedia/commons/8/86/Beat_frequency_oscillator_board.jpg)
In a
radio receiver, a beat frequency oscillator or BFO is a dedicated
oscillator used to create an audio frequency signal from
Morse code radiotelegraphy (
CW) transmissions to make them audible. The signal from the BFO is mixed with the received signal to create a
heterodyne
A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is u ...
or
beat
Beat, beats or beating may refer to:
Common uses
* Patrol, or beat, a group of personnel assigned to monitor a specific area
** Beat (police), the territory that a police officer patrols
** Gay beat, an area frequented by gay men
* Battery (c ...
frequency which is heard as a tone in the speaker. BFOs are also used to demodulate
single-sideband (SSB) signals, making them intelligible, by essentially restoring the
carrier that was suppressed at the transmitter. BFOs are sometimes included in
communications receiver
A communications receiver is a type of radio receiver used as a component of a radio communication link. This is in contrast to a ''broadcast receiver'' which is used to receive radio broadcasts. A communication receiver receives parts of the r ...
s designed for
short wave listeners; they are almost always found in communication receivers for
amateur radio, which often receive CW and SSB signals.
[Larry Wolfgang, Charles Hutchinson (ed), ''The ARRL Handbook for Radio Amateurs Sixty Eighth Edition'', ARRL, -9, pages 12-29,12-30]
The beat frequency oscillator was invented in 1901 by Canadian engineer
Reginald Fessenden
Reginald Aubrey Fessenden (October 6, 1866 – July 22, 1932) was a Canadian-born inventor, who did a majority of his work in the United States and also claimed U.S. citizenship through his American-born father. During his life he received hundre ...
. What he called the "heterodyne" receiver was the first application of the
heterodyne
A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is u ...
principle.
Overview
In
continuous wave (CW) radio transmission, also called
radiotelegraphy, or
wireless telegraphy (W/T) or
on-off keying and designated by the
International Telecommunication Union
The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
as
emission type A1A, information is transmitted by pulses of
unmodulated radio
carrier wave
In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...
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
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
on and off rapidly using a
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
called a
telegraph key
A telegraph key is a specialized electrical switch used by a trained operator to transmit text messages in Morse code in a telegraphy system. Keys are used in all forms of electrical telegraph systems, including landline (also called wire) ...
. 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 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 totally eliminated before 1950.
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
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the ...
electronic oscillator that generates a constant sine wave at a frequency ''f''
BFO that is offset from the
intermediate frequency ''f''
IF 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
In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, ''perceived'' as a periodic variation in volume whose rate is the difference of the two frequencies.
With tuning instruments that can produce ...
(
heterodyne
A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is u ...
) in the
audio
Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to:
Sound
* Audio signal, an electrical representation of sound
*Audio frequency, a frequency in the audio spectrum
* Digital audio, representation of sou ...
range results at the difference between them: ''f''
audio = , ''f''
IF - ''f''
BFO, 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 (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
A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carri ...
receiver the different frequencies of the different stations are all translated to the same
intermediate frequency (IF) by the
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
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
![D 1944 Rohde und Schwarz STT4032 Front Large](https://upload.wikimedia.org/wikipedia/commons/b/b5/D_1944_Rohde_und_Schwarz_STT4032_Front_Large.jpg)
A receiver is tuned to a
Morse code signal, and the receiver's
intermediate frequency (IF) is ''f''
IF = 45000 Hz. That means the dits and dahs have become pulses of a 45000 Hz signal, which is inaudible.
To make them audible, the frequency needs to be shifted into the audio range, for instance ''f''
audio = 1000 Hz. To achieve that, the desired BFO frequency is ''f''
BFO'' = 44000 or 46000 Hz.
When the signal at frequency ''f''
IF is mixed with the BFO frequency in the
detector
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
stage of the receiver, this creates two other frequencies or
heterodyne
A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is u ...
s: , ''f''
IF − ''f''
BFO, , and , ''f''
IF + ''f''
BFO, . The ''difference frequency'', ''f''
audio = , ''f''
IF − ''f''
BFO, = 1000 Hz, is also known as the
beat frequency
In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, ''perceived'' as a periodic variation in volume whose rate is the difference of the two frequencies.
With tuning instruments that can produce ...
.
The other, the ''sum frequency'', ''(F
if + F
bfo)'' = 89000 or 91000 Hz, is unneeded. It can be removed by a
lowpass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter ...
, such as the radio's speaker, which cannot vibrate at such a high frequency.
''f''
BFO = 44000 or 46000 Hz produces the desired 1000 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
In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude m ...
(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.
For single sideband reception, the BFO frequency is adjusted above or below the receiver intermediate frequency, depending on which sideband is used.
[
]
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. 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.[Frank Spitzer, Barry Howarth, ''Principles of Modern Instrumentation'', Holt, Rinehart and Winston, 1972, {{ISBN, 0-03-080208-3, page 98]
References
Further reading
"Radiotelephone"
''NEETS, Module 17--Radio-Frequency Communication Principles''. Integrated Publishing, Electrical Engineering Training Series.
"Voice Modes"
ARRL
Communication circuits
Electronic oscillators
Amateur radio
Electronic design
Radio electronics