Single Sideband Systems
When an audio signal of 5 kHz is used to amplitude-modulate a carrier, the output of the transmitter contains sideband frequencies in addition to the carrier frequency. The upper sideband frequencies extend to 5 kHz higher than the carrier, and the lower sideband frequencies extend to 5 kHz lower than the carrier. In normal AM broadcasts both sidebands are transmitted, requiring a bandwidth in the frequency spectrum of 10 kHz, centered on the carrier frequency. The audio signal, however, is contained in and may be retrieved from either the upper or lower sideband. Furthermore, the carrier itself contains no useful information. Therefore, the only part that needs to be transmitted is one of the sidebands. A system designed to do this is called a single sideband suppressed carrier (abbreviated SSBSC, or SSB for short). This is an important system because it requires only half of the bandwidth needed for ordinary AM, thus allowing more channels to be assigned in any given portion of the frequency spectrum. Also, because of the reduced power requirements, a 110-watt SSB transmitter may have a range as great as that of a 1,000-watt conventional AM transmitter. Almost all ham radios, commercial radiotelephones, and marine-band radios, as well as citizens band radios, use SSB systems. Receivers for such systems are more complex, however, than those for other systems. The receiver must reinsert the nontransmitted carrier before successful heterodyning can take place.
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