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

5.4 Demodulation of FM Signals 23 1
Thus, we have
fw
= T"2 ( 13.826 x 10 7 )
No integer n2 will lead to a realizablefw-
Thus, the final design is M1 = 16, M2 = 128, andf w
= 440 kHz.
5.4 DEMODULATION OF FM SIGNALS
The information in an FM signal resides in the instantaneous frequency Wi = w, + k r
m(t).
Hence, a frequency-selective network with a transfer function of the form IH (J) I = 2anf + b
over the FM band would yield an output proportional to the instantaneous frequency
(Fig. 5.12a). * There are several possible circuits with such characteristics. The simplest among
them is an ideal differentiator with the transfer functionj2rrf.
Figure 5.12
(a) FM
demodulator
frequency
response.
(b) Output of a
differentiator to
the input FM
wave. (c) FM
demodulation by
direct
differentiation.
t
LF:J
w-
band
(a)
A [we + k 1
m(t)]
',
/
/
....
/ .... ,
" /
,_
/
/ '- /
(b)
..,.
<pFM (t) <pFM
d (t) Envelope
dt
I detector
( c)
A[wc +
* Provided the variations of w; are slow in comparison to the time constant of the network.