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

4.2 Double-Sideband Amplitude Modulation 145
The message spectrum consists of two impulses located at ±1 m, as shown in Fig. 4.2a.
The DSB-SC (modulated) spectrum, as seen from Eq. (4.1), is the baseband spectrum in
Fig. 4.2a shifted to the right and the left by l e (times one-half), as shown in Fig. 4.2b.
This spectrum consists of impulses at angular frequencies ±U c - lm) and ± Uc + lm).
The spectrum beyond le is the USB, and the one below l e is the LSB. Observe that the
DSB-SC spectrum does not have the component of the carrier frequency le- This is why
it is called suppressed carrier.
In the time domain approach, we work directly with signals in the time domain. For the
baseband signal m(t) = cos W m t, the DSB-SC signal (/JDSB-sdt) is
(/JDSB - sdt) = m(t) cos W e t
This shows that when the baseband (message) signal is a single sinusoid of frequency lm,
the modulated signal consists of two sinusoids: the component of frequency l e + lm (the
USB) and the component of frequency l e -lm (the LSB). Figure 4.2b shows precisely the
spectrum of <p 0sa-sc (t). Thus, each component of frequency lm in the modulating signal
turns into two components of frequencies le+ lm and l e -lm in the modulated signal. Note
the curious fact that there is no component of the carrier frequency le on the right-hand
side of the preceding equation. As mentioned, this is why it is called double-sideband
suppressed carrier (DSB-SC) modulation.
We now verify that the modulated signal (/JDSB-sdt) = cos W m t cos W e t, when applied
to the input of the demodulator in Fig. 4. le, yields the output proportional to the desired
baseband signal cos w m t. The signal e(t) in Fig. 4.le is given by
e(t) = cos W m t cos 2 W e t
1
= 2
COS W m t (1 + COS 2w c t)
Figure 4.2
Example of
DSB-SC
modulation.
M(f)
'" l I , 1 "'
(a)
f-
'"l r
USB LSB
DSB spectrum
I,
(b)
'"l r
LSB USB
f-
- 2 fc
114 1/4
f-
1/8 !
I
2fc
1/8
(c)