0195136047.pdf

15.2 ANALOG COMMUNICATION SYSTEMS 689

EXAMPLE 15.2.1

Letf(t) be a sinusoid given byf(t)=Amcosωmtwith periodTm= 2 π/ωm. Apply Equation
(15.2.6) and obtain the value ofηAM.

Solution

f^2 (t)=

1

Tm

∫Tm/ 2

−Tm/ 2

A^2 mcos^2 (ωmt) dt

=

A^2 m

8 π

∫ 2 π

− 2 π

( 1 +cosx) dx=

A^2 m

2

ηAM=

A^2 m

2 A^2 c+A^2 m

=

(Am/Ac)^2

2 +(Am/Ac)^2

For no overmodulation,Am≤Ac, so thatηAM≤^1 / 3.

Suppressed-carrier AM,also known as double-sideband suppressed-carrier (DSB SC AM),
results when the carrier termAcin Equation (15.2.3) is eliminated. The DSB waveform is then
given by

sDSB(t)=f(t)cos(ωct+φc) (15.2.7)

Figure 15.2.4(a) depicts the waveforms of the message signal and its amplitude spectrum; Figure
15.2.4(b) shows the DSB waveformsDSB(t), and Figure 15.2.4(c) displays the amplitude spectrum
ofsDSB(t). From Equation (15.2.6) withAc=0, the efficiency of DSB SC AM comes out as 1.0,
or 100%. While the power efficiency is increased, there will be added complexity, especially in
the demodulator.

f(t)

sDSB(t)

Amplitude spectrum of sDSB(t)

USB

ω ω

−ωc c

LSB LSB

0

USB

t

t

ω

00

Message signal

Phase reversal when f(t) changes sign

(a)

(b)

(c)

USB

Amplitude spectrum

LSB

Figure 15.2.4(a)Message signal and its

amplitude spectrum.(b)DSB SC sig-

nalsDSB(t) corresponding to the message.

(c)Corresponding SC AM signal spectrum.