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

4.4 Bandwidth-Efficient Amplitude Modulations
163
Fi g ure 4. 16
SS B spectra for
tone modulation.
(a)
(
h)
t
i t
-Uc + fm) - fc -Uc - fm)
-fm O fm
1-
I
DSB sp m Wm
t t
I
0 Uc - fm) f Uc + fm)
1-
(c)
t
I
0
USB speco,,m
t
Uc + fm)
1-
(d)
t
-Uc - fm)
0 Uc -fm) J-
Finally, the coherent demodulation of the SSB tone modulation is can be achieved by
cpsss (t)2 cos W e t= 2cos (ev e ± W m )tcos W e t
= COS W m t + COS (we + W m )t
which can be sent to a lowpass filter to retrieve the message tone cos W m t.
SSB Modulation Systems
Three methods are commonly used to generate SSB signals: phase shifting, selective filtering,
and the Weaver method. 1 None of these modulation methods are precise, and all generally
require that the baseband signal spectrum have little power near the origin.
The phase shift method directly uses Eq. ( 4.20) as its basis. Figure 4.17 shows its implementation.
The box marked "-n /2" is a phase shifter, which delays the phase of every positive
spectral component by n /2. Hence, it is a Hilbert transformer. Note that an ideal Hilbert phase
shifter is unrealizable. This is because the Hilbert phase shifter requires an abrupt phase change
of n at zero frequency. When the message m(t) has a de null and very little low-frequency
content, the practical approximation of this ideal phase shifter has almost no real effect and
does not affect the accuracy of SSB modulation.
In the selective-filtering method, the most commonly used method of generating SSB
signals, a DSB-SC signal is passed through a sharp cutoff filter to eliminate the undesired sideband.
To obtain the USB, the filter should pass all components above frequency le unattenuated
and completely suppress all components below l e - Such an operation requires an ideal filter,
which is unrealizable. It can, however, be approximated closely if there is some separation