Page 197 - Fundamentals of Communications Systems

Amplitude Modulation 6.19

Im H ()]
Q [
() 1
1 f
f
Q [
−f v f v Im H ()]
() 2
f
−1

Figure 6.21 Examples of the imaginary part of the
quadrature filter for VSB-AM.

We want

H z (f ) = [1 + jH Q (f )] = 0 − W ≤ f ≤− f v (6.19)

Since H Q (f ) =
[H Q (f )] + j [H Q (f )] it is simple to see that Eq. (6.19) implies
that

[H Q (f )] = 1
[H Q (f )] = 0 − W ≤ f ≤− f v (6.20)
Note, since h Q (t) is real then this implies

[H Q (f )] =−1
[H Q (f )] = 0 f v ≤ f ≤ W (6.21)
but the values for H Q (f ), − f v ≤ f ≤ f v are unconstrained. A plot of two
possible realizations of the filter H Q (f ) is shown in Figure 6.21. It should be
noted that our discussion focused on eliminating a portion of the lower sideband
but similar results hold for eliminating the upper sideband.

EXAMPLE 6.8
Analog television broadcast in the United States uses a video signal with a bandwidth of
approximately 4.5 MHz. The transmitted signals are a form of quadrature modulation,
where f v = 1.25 MHz for a total channel bandwidth of <6 MHz. Television stations in
the United States have 6 MHz spacings. Consequently
4.5 MHz
E B = = 75%
6 MHz

6.3.2 Single Sideband AM
An interesting special case of VSB-AM occurs when f v → 0. In this case the
E B → 100%. This is accomplished by eliminating one of the sidebands and
hence this modulation is often termed single sideband amplitude modulation