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

4. 9 MATLAB Exercises 183
Fi g ure 4.32
Time domain
signals during
DSB-SC
modulation and
demodulation.
Message signal
2 --------------
DSB-SC modulated signal
-I
-2 ---------'
-0.02 -0.01 0 0.01 0.02
t(sec)
-0.02 -0.01 0 0.01
t(sec)
0.02
e(t)
Recovered signal
2 ------------- 2 ---------------
' 0
-1
-2 '---------L-----'
-0.02 -0.01 0 0.01 0.02
t(sec)
-0.02 -0.01
0 0.01
t(sec)
0.02
xlabel ('{\it t} (sec ) '); ylabel ('{\it m} ( {\it t} ) ')
subplot (223) ; td2 =plot (t,s_dsb) ;
axis ( Trange ) ; set (td2 , 'Linewidth ' , 2) ;
xlabel ('{\it t} (sec ) '); ylabel ('{\it s}_{\rm DSB} ({\ it t} ) ')
Frange= [-600 600 0 200]
subplot (222) ;fdl=plot ( freqm, abs (M_fre) ) ;
axis ( Frange ) ; set (fdl , 'Linewidth ' , 2) ;
xlabel ('{\it f } (Hz) '); ylabel ('{\it M} ( {\it f}) ')
subplot (224) ;fd2 =plot ( freqs ,abs (S_dsb) ) ;
axis ( Frange ) ; set (fd2 , 'Linewidth ' , 2) ;
xlabel ('{\it f} (Hz) '); ylabel ('{\it S}_{rm DSB} ( {\it f} ) ')
The first modulation example, ExampleDSBdemf il t. mis based on a strictly low-pass
message signal mo(t). Next, we will generate a different message signal that is not strictly
band-limited. In effect, the new message signal consists of two triangles:
mi (t) = t:, ( t + 0.01 ) - t:, ( t-0.01 )
0.01
0.01
Coherent demodulation is also implemented with a finite impulse response (FIR) low-pass
filter of order 40. The original message signal m(t), the DSB-SC signal m(t) cos W e t, the
demodulator signal e(t) = m(t) cos 2 w e t, and the recovered message signal md (t) after lowpass
filtering are all given in Fig. 4.32 for the time domain and in Fig. 4.33 for the frequency
domain. The low-pass filter at the demodulator has bandwidth of 150 Hz. The demodulation
result shows almost no distortion.