Inradio communications, asideband is aband offrequencies higher than or lower than thecarrier frequency, that are the result of themodulation process. The sidebands carry the information transmitted by the radio signal. The sidebands comprise all thespectral components of the modulated signal except the carrier. The signal components above the carrier frequency constitute theupper sideband (USB), and those below the carrier frequency constitute thelower sideband (LSB). All forms of modulation produce sidebands.

We can illustrate the creation of sidebands with one trigonometric identity:

${\displaystyle \cos (A)\cdot \cos (B)\equiv \frac{1}{2}\cos (A+B)+\frac{1}{2}\cos (A-B)}$

Adding${\displaystyle \cos (A)}$ to both sides:

${\displaystyle \cos (A)\cdot [1+\cos (B)]=\frac{1}{2}\cos (A+B)+\cos (A)+\frac{1}{2}\cos (A-B)}$

Substituting (for instance)  ${\displaystyle A\triangleq 1000\cdot t}$  and  ${\displaystyle B\triangleq 100\cdot t,}$  where${\displaystyle t}$ represents time:

${\displaystyle \underset{\text{carrier wave}}{\underbrace{\cos (1000t)}}\cdot \underset{\text{amplitude modulation}}{\underbrace{[1+\cos (100t)]}}=\underset{\text{upper sideband}}{\underbrace{\frac{1}{2}\cos (1100t)}}+\underset{\text{carrier wave}}{\underbrace{\cos (1000t)}}+\underset{\text{lower sideband}}{\underbrace{\frac{1}{2}\cos (900t)}}.}$

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.^[1]

In the example above, across-correlation of the modulated signal with a pure sinusoid,${\displaystyle \cos (\omega t),}$ is zero at all values of${\displaystyle \omega }$ except 1100, 1000, and 900. And the non-zero values reflect the relative strengths of the three components. A graph of that concept, called aFourier transform (orspectrum), is the customary way of visualizing sidebands and defining their parameters.

Amplitude modulation of acarrier 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 900 kHz carrier is amplitude modulated by a 1 kHz audio signal, there will be components at 899 kHz and 901 kHz as well as 900 kHz in the generatedradio frequency spectrum; so anaudiobandwidth of (say) 7 kHz will require aradio spectrum bandwidth of 14 kHz. In conventional AMtransmission, as used bybroadcast band AM stations, the original audio signal can be recovered ("detected") by eithersynchronous detector circuits or by simpleenvelope detectors because the carrier and both sidebands are present. This is sometimes calleddouble 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 termDSB reduced-carrier normally implies enough carrier remains in the transmission to enable areceiver circuit to regenerate a strong carrier or at leastsynchronise aphase-locked loop but there are forms where the carrier is removed completely, producingdouble sideband withsuppressed 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 thestereophonic difference (L-R) information transmitted in stereoFM broadcasting on a 38 kHzsubcarrier where a low-power signal at half the 38-kHz carrier frequency is inserted between the monaural signal frequencies (up to 15 kHz) and the bottom of the stereo information sub-carrier (down to 38–15 kHz, i.e. 23 kHz). The receiver locally regenerates the subcarrier by doubling a special 19 kHzpilot tone. In another example, thequadrature modulation used historically for chroma information inPAL 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 aCostas loop orsquaring loop. This is common in digital transmission systems such asBPSK where the signal is continually present.

If part of one sideband and all of the other remain, it is calledvestigial sideband, used mostly withtelevisionbroadcasting, which would otherwise take up an unacceptable amount ofbandwidth. Transmission in which only one sideband is transmitted is calledsingle-sideband modulation or SSB. SSB is the predominant voice mode onshortwave radio other thanshortwave broadcasting. Since the sidebands are mirror images, which sideband is used is a matter of convention.

In SSB, thecarrier is suppressed, significantly reducing theelectrical power (by up to 12 dB) without affecting the information in the sideband. This makes for more efficient use of transmitter power and RF bandwidth, but abeat frequency oscillator must be used at thereceiver 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 frequencytransposer: 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 also generates sidebands, the bandwidth consumed depending on themodulation 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.

Sidebands caninterfere withadjacent channels. The part of the sideband that would overlap the neighboring channel must be suppressed byfilters, before or after modulation (often both). Inbroadcast bandfrequency modulation (FM),subcarriers above 75 kHz are limited to a smallpercentage of modulation and are prohibited above 99 kHz altogether to protect the ±75 kHz normaldeviation and ±100 kHzchannel 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 thefuzzbox musical effect.

• Independent sideband
• Out-of-band communications involve a channel other than the main communication channel.
• Side lobe
• Sideband computing is a distributed computing method using a channel separate from the main communication channel.
• TV transmitter

• Amateur radio

• Articles with short description
• Short description is different from Wikidata
• Wikipedia articles incorporating text from the Federal Standard 1037C
• Wikipedia articles incorporating text from MIL-STD-188