In
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transm ...
communications, a sideband is a
band of
frequencies higher than or lower than the
carrier frequency
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 ...
, that are the result of the
modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands comprise all the
spectral components of the modulated signal except the carrier. The signal components above the carrier frequency constitute the upper sideband (USB), and those below the carrier frequency constitute the lower sideband (LSB). All forms of modulation produce sidebands.
Sideband creation
We can illustrate the creation of sidebands with one trigonometric identity:
:
Adding
to both sides:
:
Substituting (for instance)
and
where
represents time:
:
Adding more complexity and time-variation to the amplitude modulation also adds it to the sidebands, causing them to widen in bandwidth and change with time. In effect, the sidebands "carry" the information content of the signal.
[Tony Dorbuck (ed.), ''The Radio Amateur's Handbook, Fifty-Fifth Edition'', American Radio Relay League, 1977, p. 368]
Sideband Characterization
In the example above, a
cross-correlation of the modulated signal with a pure sinusoid,
is zero at all values of
except 1100, 1000, and 900. And the non-zero values reflect the relative strengths of the three components. A graph of that concept, called a
Fourier transform (or ''spectrum''), is the customary way of visualizing sidebands and defining their parameters.
Amplitude modulation
Amplitude modulation of a
carrier signal normally results in two mirror-image sidebands. The signal components above the carrier frequency constitute the upper sideband (USB), and those below the carrier frequency constitute the lower sideband (LSB). For example, if a 900kHz carrier is amplitude modulated by a 1kHz audio signal, there will be components at 899kHz and 901kHz as well as 900kHz in the generated
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 uppe ...
spectrum; so an
audio bandwidth of (say) 7kHz will require a
radio spectrum bandwidth of 14kHz. In conventional AM
transmission, as used by ''broadcast band'' AM stations, the original audio signal can be recovered ("detected") by either
synchronous detector circuits or by simple
envelope detectors because the carrier and both sidebands are present. This is sometimes called double sideband amplitude modulation (DSB-AM), but not all variants of DSB are compatible with envelope detectors.
In some forms of AM, the carrier may be reduced, to save power. The term
''DSB reduced-carrier'' normally implies enough carrier remains in the transmission to enable a
receiver circuit to regenerate a strong carrier or at least
synchronise a
phase-locked loop but there are forms where the carrier is removed completely, producing
double sideband with ''suppressed'' carrier (DSB-SC). Suppressed carrier systems require more sophisticated circuits in the receiver and some other method of deducing the original carrier frequency. An example is the
stereophonic difference (L-R) information transmitted in stereo
FM broadcasting
FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is c ...
on a 38 kHz
subcarrier where a low-power signal at half the 38-kHz carrier frequency is inserted between the monaural signal frequencies (up to 15kHz) and the bottom of the stereo information sub-carrier (down to 38–15kHz, i.e. 23kHz). The receiver locally regenerates the subcarrier by doubling a special 19 kHz
pilot tone. In another example, the
quadrature modulation used historically for chroma information in
PAL television broadcasts, the synchronising signal is a short burst of a few cycles of carrier during the
"back porch" part of each scan line when no image is transmitted. But in other DSB-SC systems, the carrier may be regenerated directly from the sidebands by a
Costas loop or
squaring loop
Squaring may refer to:
*Square (algebra)
In mathematics, a square is the result of multiplication, multiplying a number by itself. The verb "to square" is used to denote this operation. Squaring is the same as exponentiation, raising to the po ...
. This is common in digital transmission systems such as
BPSK where the signal is continually present.
If part of one sideband and all of the other remain, it is called
vestigial sideband, used mostly with
television
Television, sometimes shortened to TV, is a telecommunication Media (communication), medium for transmitting moving images and sound. The term can refer to a television set, or the medium of Transmission (telecommunications), television tra ...
broadcasting
Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began ...
, which would otherwise take up an unacceptable amount of
bandwidth. Transmission in which only one sideband is transmitted is called
single-sideband modulation or SSB. SSB is the predominant voice mode on
shortwave radio other than
shortwave broadcasting. Since the sidebands are mirror images, which sideband is used is a matter of convention.
In SSB, the
carrier is suppressed, significantly reducing the
electrical power (by up to 12dB) without affecting the information in the sideband. This makes for more efficient use of transmitter power and RF bandwidth, but a
beat frequency oscillator must be used at the
receiver to reconstitute the carrier. If the reconstituted carrier frequency is wrong then the output of the receiver will have the wrong frequencies, but for speech small frequency errors are no problem for intelligibility. Another way to look at an SSB receiver is as an RF-to-audio frequency
transposer: in USB mode, the dial frequency is subtracted from each radio frequency component to produce a corresponding audio component, while in LSB mode each incoming radio frequency component is subtracted from the dial frequency.
Frequency modulation
Frequency modulation
Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and Run-length limited#FM: .280. ...
also generates sidebands, the bandwidth consumed depending on the modulation index - often requiring significantly more bandwidth than DSB.
Bessel functions can be used to calculate the bandwidth requirements of FM transmissions.
Carson's rule is a useful approximation of bandwidth in several applications.
Effects
Sidebands can
interfere with
adjacent channels. The part of the sideband that would overlap the neighboring channel must be suppressed by
filters, before or after modulation (often both). In
broadcast band frequency modulation
Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The technology is used in telecommunications, radio broadcasting, signal processing, and Run-length limited#FM: .280. ...
(FM),
subcarriers above 75
kHz are limited to a small
percentage of modulation and are prohibited above 99 kHz altogether to protect the ±75 kHz normal
deviation and ±100 kHz
channel boundaries.
Amateur radio and public service FM transmitters generally utilize ±5 kHz deviation.
To accurately reproduce the modulating waveform, the entire signal processing path of the system of transmitter, propagation path, and receiver must have enough bandwidth so that enough of the sidebands can be used to recreate the modulated signal to the desired degree of accuracy.
In a non-linear system such as an amplifier, sidebands of the original signal frequency components may be generated due to distortion. This is generally minimized but may be intentionally done for the
fuzzbox musical effect.
See also
*
Independent sideband
Independent sideband (ISB) is an AM single sideband mode which is used with some AM radio transmissions. Normally each sideband carries identical information, but ISB modulates two different input signals — one on the upper sideband, ...
*
Out-of-band communications involve a channel other than the main communication channel.
*
Side lobe
*
Sideband computing
Sideband computing is where a user connects to some normal network service, and a separate communication channel is opened through which a server distributes tasks to the clients. Through sideband computing method, any network server which has a l ...
is a distributed computing method using a channel separate from the main communication channel.
*
TV transmitter
References
*
*
Department of The Army
The United States Department of the Army (DA) is one of the three military departments within the Department of Defense of the U.S. The Department of the Army is the federal government agency within which the United States Army (U.S.) is or ...
Technical Manual TM 11-685 "Fundamentals of Single Sideband Communications"
{{Authority control
Amateur radio