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

246 ANGLE MODULATION AND DEMODULATION
Figure 5,21
FM modulation
and
demodulation:
(a) original
message;
(b) recovered
signal.
0.5
0
-0.5
Message signal
-1
--0.04 --0.03 --0.02 --0.01 0 0.01 0.02 0.03 0.04
t (sec)
Demodulated FM signal
3
i!
0.5
o----------------------
--0.04 -0.03 --0.02 --0.01 0 0.01 0.02 0.03 0.04
t (sec)
Upon applying the rectifier for envelope detection, we see that the message signal follows
closely to the envelope variation of the rectifier output.
Finally, the rectifier output signal is passed through a low-pass filter with bandwidth
100 Hz. We used the finite impulse response low-pass filter of order 80 this time because of
the tighter filter constraint in this example. The FM detector output is then compared with the
original message signal in Fig. 5.21.
The FM demodulation results clearly show some noticeable distortions. First, the higher
order low-pass filter has a much longer response time and delay. Second, the distortion during
the negative half of the message is more severe because the rectifier generates very few
cycles of the half-sinusoid. This happens because when the message signal is negative, the
instantaneous frequency of the FM signal is low. Because we used a carrier frequency of only
300 Hz, the effect of low instantaneous frequency is much more pronounced. If a practical
carrier frequency of 100 MHz were applied, this kind of distortion would be completely
negligible.
REFERENCES
1. J. Carson, '"Notes on the Theory of Modulation," Proc. IRE, vol. 10, pp. 57-64, Feb. 1922.
2. J. Carson, '"Reduction of Atmospheric Disturbances," Proc. IRE, vol. 16, July 1928.
3. E. H. Armstrong, "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency
Modulation," Proc. IRE, vol. 24, pp. 689-740, May 1936.
4. "A Revolution in Radio," Fortune, vol. 20, p. 116, Oct. 1939.
5. L. Lessing, Man of High Fidelity: Edwin Howard Armstrong, Lippincott, Philadelphia, 1956.