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Modulation

Modulation 63

low-cost transmitters and receivers for single-sideband (SSB) communications
have become quite popular, and have completely replaced the older AM long-
range voice communication systems.

2.3.2 Fundamentals
Single-sideband suppressed carrier is a form of AM, but transmits only a sin-
gle sideband, rather than the two sidebands and the complete carrier of ampli-
tude modulation. In SSB the carrier, which holds no information, and the
other sideband, which duplicates the information present in the transmitted
sideband, are strongly attenuated (Fig. 2.16). Since one of the sidebands is
attenuated, SSB requires only half the bandwidth that AM consumes for its
transmissions, which also translates into less noise received (Fig. 2.17).
Although amplitude modulation’s fading characteristics are quite poor, in SSB
fading is much less of a problem. This is because the multiple phase depen-
dencies between all of AM’s transmitted elements—both sidebands and the
carrier—need not be sustained in SSB, inasmuch as only a single sideband is
actually being transmitted. Power efficiency is also much higher in SSB than
in AM because of the power savings in transmitting only a single sideband,
while further power is conserved since a transmitted signal is produced only
when the baseband is actually present at the modulator.

Figure 2.16 A single-sideband
signal.

Figure 2.17 A narrower received
bandwidth means less noise.

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