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

376 PRINCIPLES OF DIGITAL DATA TRANSMISSION
Figure 7.32
PSD of (a) ASK,
(b) PSK, and
(c) FSK.
0 fc f -
(a)
0
(b)
fc 1-
( c)
Of course, we can also modulate bipolar, or any other scheme discussed earlier. Also, note
that the use of the NRZ rectangular pulse in Fig. 7 .30 or 7 .31 is for the sake of illustration only.
In practice, baseband pulses may be spectrally shaped to eliminate ISL
7.8.3 Connections between Analog and Digital
Carrier Modulations
There is a natural and clear connection between ASK and AM because the message information
is directly reflected in the varying amplitude of the modulated signals. Because of its
nonnegative amplitude, ASK is essentially an AM signal with modulation index µ = I. There
is a similar connection between FSK and FM. FSK is simply an FM signal with only limited
number of instantaneous frequencies.
The connection between PSK and analog modulation is a bit more subtle. For PSK, the
modulated signal can be written as
<P PsK (t) = A cos (wet + 0 k )
It can therefore be connected with PM. However, a closer look at the PSK signal reveals that
because of the constant phase 0 k , its instantaneous frequency, in fact, does not change. In fact,