690 COMMUNICATION SYSTEMS
A relatively simple method to generate a DSB SC AM signal is by employing two (identical)
conventional AM modulators, such as two square-law AM modulators, arranged in the config-
uration of Figure 15.2.5, which is known as abalanced modulator.φc=0 is assumed here for
simplicity.
Single-sideband (SSB) AMis obtained by filtering one sideband of the DSB signal. As pointed
out earlier, a DSB SC AM signal required a channel bandwidth of 2Wfor transmission, where
Wis the bandwidth of the baseband signal. However, the two sidebands are redundant. The
transmission of either sideband is sufficient to reconstruct the message signal at the receiver.
Thus, the bandwidth of the transmitted signal is reduced to that of the baseband signal. Figure
15.2.6 illustrates the generation of an SSB AM signal by filtering one of the sidebands (either
USB or LSB) of a DSB SC AM signal. SSB is popular with amateur radio operators because of
its high efficiency(ηSSB= 1 )and bandwidth savings.
Vestigial-sideband (VSB) AMis a variation of SSB where a small portion (or vestige) of
the filtered sideband is allowed to remain. The stringent frequency-response requirements on the
sideband filter in an SSB AM system can be relaxed by allowing a part, known as a vestige, of
the unwanted sideband to appear at the output of the modulator. Thus, the design of the sideband
filter is simplified at the cost of a modest increase in the channel bandwidth required to transmit
the signal. Figure 15.2.7 illustrates the generation of a VSB AM signal. VSB is mainly used in theAc cos 2πfctAc [1 + x(t)] cos 2πfctAc [1 − x(t)] cos 2πfct2 Acx(t) cos 2πfctAM
modulatorAM
modulator−x(t)x(t)
Message signal+
−Figure 15.2.5Block diagram of
a balanced modulator.cos (ωct + φc)Balanced
modulatorHigh-pass
filter H(ω)Gain = 20 02−ωc 0 ωc −ωc ωc −ωc ωcf(t) sDSB(t)SDSB(ω) H(ω) SSSB(ω)ωω ωsSSB(t)
Message
signalFigure 15.2.6Generation of a SSB AM signal by filtering one of the sidebands of a
DSB SC AM signal.