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

186 AMPLITUDE MODULATIONS AND DEMODULATIONS
Figure 4.34
Time domain
signals in AM
modulation and
noncoherent
demodulation.
Message signal
0 0
AM modulated signal
-1
-1
-2 -2 '
-0.02 -0.01 0 0.01 0.02 -0.02 --0.01 0 0.01 0.02
t(sec) t(sec)
2
Rectified signal without local carrier
Detected signal
3 0 "'
-1
-0
f
0.5
0
-2
--0.01 0 0.01 0.02
--0.5
-0.02 -0.01 0 0.01 0.02
t(sec)
t(sec)
-0.02
the message signal, the corresponding AM signal, the rectified signal in noncoherent demodulation,
and the rectified signal after passing through a low-pass filter. The low-pass filter at
the demodulator has a bandwidth of 150 Hz. The signals in the time domain are shown in
Fig. 4.34, whereas the corresponding frequency domain signals are shown in Fig. 4.35.
Notice the large impulse in the frequency domain of the AM signal. The limited time
window means that no ideal impulse is possible and only very large spikes centered at the
carrier frequency of ±300 Hz are visible. Finally, because the message signal bandwidth is
not strictly band-limited, the relatively low carrier frequency of 300 Hz forces the low-pass
filter at the demodulator to truncate some with the message component in the demodulator.
Distortion near the sharp comers of the recovered signal is visible.
% (ExampleAMdemf ilt.m)
% This program uses triangl .m to illustrate AM modulation
% and demodulation
ts=l .e-4 ;
t=-0 .04:ts:0.04;
Ta=0 .01; fc=500;
m_s ig=triangl ((t+0.01) /0.01)-triangl ((t-0 .01)/0.01) ;
Lm_s ig=length (m_sig) ;
Lfft=length (t) ; Lfft=2 A ceil ( log2 (Lfft) );
M_fre=fftshi ft ( fft(m_sig, Lfft) );
freqm= (-Lfft/2:Lfft/2-1)/ (Lfft*ts) ;
B_m= 150; %Bandwidth of the signal is B_m Hz .
h=fir1 (40, [B_m*ts));