B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

4.2 Double-Sideband Amplitude Modulation 143
Fi g
ure 4. 1
DSB-SC
modulation and
demodulation.
m(t)
(Modulating signal)
m(t) cos OV
(Modulated signal)
cos ro c t
(Carrier)
( a) Modulator
m(t)
(b)
-B B
f
m(t) '
...., .,- ---- ' ,
m(t) cos av
-- ---- ------ A LSB
/
( c)
-1;
:----- 2B _____,;
0
le
f---
-m(t)
-'2fc
I--
m(t) cos ov
.l m(t)
2
(e) Demodulator
COS Wcf
(Carrier)
broadcast band AM radio, for instance, with B = 5 kHz and the band of 550 to 1600 kHz for
the carrier frequencies, gives a ratio offc/B roughly in the range of 100 to 300.
Demodulation
The DSB-SC modulation translates or shifts the frequency spectrum to the left and the right
by f (i.e., at +Jc and -J c ), as seen from Eq. (4. 1). To recover the original signal m(t) from the
modulated signal, it is necessary to retranslate the spectrum to its original position. The process
of recovering the signal from the modulated signal (retranslating the spectrum to its original
position) is referred to as demodulation. Observe that if the modulated signal spectrum in
Fig. 4.lc is shifted to the left and to the right by f c (and multiplied by one-half), we obtain the