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

5.3 Generating FM Waves 223
Figure 5.8
(a) Narrowband
PM generator.
(b) Narrowband
FM signal
generator.
m(t)
DSB-SC
modulator
- Ak p
m(t) sin wJ
DSB-SC
modulator
(a)
(b)
Figure 5.9
(a) Hard limiter
and bandpass
filter used to
remove
amplitude
variations in FM
wave. (b) Hard
limiter inputoutput
characteristic.
(c) Hard limiter
input and the
corresponding
output. (d) Hard
limiter output as
a function of 0.
A(t ) cos [w e
t+ 0(1)]
Hard
limiter
---- Bandpass
filter
(a)
V; -+-
4
-;- cos [w t + 0(t)]
e
(b)
(c)
-I
rr 3rr 5rr 0 -
2 T 2
(d)
Bandpass Limiter
The amplitude variations of an angle-modulated carrier can be eliminated by what is known as
a bandpass limiter, which consists of a hard limiter followed by a bandpass filter (Fig. 5.9a).
The input-output characteristic of a hard limiter is shown in Fig. 5.9b. Observe that the bandpass
limiter output to a sinusoid will be a square wave of unit amplitude regardless of the incoming
sinusoidal amplitude. Moreover, the zero crossings of the incoming sinusoid are preserved