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Single-sideband modulation
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==Basic concept== Radio transmitters work by mixing a [[radio frequency]] (RF) signal of a specific frequency, the [[carrier wave]], with the audio signal to be broadcast. In AM transmitters this mixing usually takes place in the final RF amplifier (high level modulation). It is less common and much less efficient to do the mixing at low power and then amplify it in a linear amplifier. Either method produces a set of frequencies with a strong signal at the carrier frequency and with weaker signals at frequencies extending above and below the carrier frequency by the maximum frequency of the input signal. Thus the resulting signal has a [[spectrum]] whose bandwidth is twice the maximum frequency of the original input audio signal. SSB takes advantage of the fact that the entire original signal is encoded in each of these "sidebands". Since a good receiver can extract the complete original signal from either the upper or lower sideband, it is not essential to transmit both sidebands plus the carrier. There are several methods for eliminating the carrier and one sideband from the transmitted signal. Producing this single sideband signal can be done at high level in the final amplifier stage as with AM <ref>{{cite thesis |url=http://hdl.handle.net/10945/24839 |title=Single sideband transmission by envelope elimination and restoration |author=Michael Murray Elliott |date=1953 |publisher=Naval Postgraduate School |hdl=10945/24839 |type=Thesis }}</ref> <ref>{{cite journal |url=https://ieeexplore.ieee.org/document/4051049 |title=Single-sideband transmission by envelope elimination and restoration |journal=Proceedings of the IRE |volume=40 |issue=7 |author=Leonard R Kahn |date=July 1952 |pages=803β806 |doi=10.1109/JRPROC.1952.273844 |s2cid=51669401 |url-access=subscription }}</ref> but it is usually produced at a low power level and linearly amplified. The lower efficiency of linear amplification partially offsets the power advantage gained by eliminating the carrier and one sideband. Nevertheless, SSB transmissions use the available amplifier energy considerably more efficiently, providing longer-range transmission for the same power output. In addition, the occupied spectrum is less than half that of a full carrier AM signal. SSB reception requires frequency stability and selectivity well beyond that of inexpensive AM receivers which is why broadcasters have seldom used it. In point-to-point communications, where expensive receivers are in common use already, they can successfully be adjusted to receive whichever sideband is being transmitted.
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