CCS - Analog Modulation - Amplitude Modulation (AM) - DSB-AM (Double-SideBand Amplitude Modulation)

Amplitude Modulation (AM)

Amplitude modulation (AM), which is frequently referred to as linear modulation, is
the family of modulation schemes in which the amplitude of a sinusoidal carrier is

changed as a function of the modulating signal. This class of modulation schemes
consists of DSB-AM (double-sideband amplitude modulation), conventional amplitude
modulation, SSB-AM (single-sideband amplitude modulation), and VSB-AM
(vestigial-sideband amplitude modulation). The dependence between the modulating
signal and the amplitude of the modulated carrier can be very simple, as, for example,
in the DSB-AM case, or much more complex, as in SSB-AM or VSB-AM. Amplitudemodulation
systems are usually characterized by a relatively low bandwidth requirement
and power inefficiency in comparison to the angle-modulation schemes. The
bandwidth requirement for AM systems varies between W and 2W, where W denotes
the bandwidth of the message signal. For SSB-AM the bandwidth is W, for DSBAM
and conventional AM the bandwidth is 2W, and for VSB-AM the bandwidth
is between W and 2W. These systems are widely used in broadcasting (AM radio
and TV video broadcasting), point-to-point communication (SSB), and multiplexing
applications (for example, transmission of many telephone channels over microwave
links).

DSB-AM

A typical message spectrum and the spectrum of the corresponding DSB-AM modulated
signal are shown in Figure 3 .1.

Matlab Coding

 1 % MATLAB script for Illustrative Problem 3.1
 2 % Demonstration script for DSB-AM. The message signal is
 3 % +1 for 0 < t < t0/3, -2 for t0/3 < t < 2t0/3, and zero otherwise.

 4 echo on
 5 t0=.15;                                 % signal duration
 6 ts=0.001;                               % sampling interval
 7 fc=250;                                 % carrier frequency
 8 snr=20;                                 % SNR in dB (logarithmic)
 9 fs=1/ts;                                % sampling frequency
10 df=0.3;                                 % desired freq. resolution
11 t=[0:ts:t0];                            % time vector
12 snr_lin=10^(snr/10);                    % linear SNR

13 % message signal
14 m=[ones(1,t0/(3*ts)),-2*ones(1,t0/(3*ts)),zeros(1,t0/(3*ts)+1)];
15 c=cos(2*pi*fc.*t);                      % carrier signal
16 u=m.*c;                                 % modulated signal
17 [M,m,df1]=fftseq(m,ts,df);              % Fourier transform
18 M=M/fs;                                 % scaling
19 [U,u,df1]=fftseq(u,ts,df);              % Fourier transform
20 U=U/fs;                                 % scaling
21 [C,c,df1]=fftseq(c,ts,df);              % Fourier transform
22 f=[0:df1:df1*(length(m)-1)]-fs/2;       % freq. vector
23 signal_power=spower(u(1:length(t)));    % power in modulated signal
24 noise_power=signal_power/snr_lin;       % Compute noise power.
25 noise_std=sqrt(noise_power);            % Compute noise standard deviation.
26 noise=noise_std*randn(1,length(u));     % Generate noise.
27 r=u+noise;                % Add noise to the modulated signal.
28 [R,r,df1]=fftseq(r,ts,df);       % spectrum of the signal+noise
29 R=R/fs;                                 % scaling
30 pause  % Press a key to show the modulated signal power.
31 signal_power
32 pause  % Press any key to see a plot of the message.
33 clf
34 subplot(2,2,1)
35 plot(t,m(1:length(t)))
36 xlabel('Time')
37 title('The message signal')
38 pause  % Press any key to see a plot of the carrier.
39 subplot(2,2,2)
40 plot(t,c(1:length(t)))
41 xlabel('Time')
42 title('The carrier')
43 pause  % Press any key to see a plot of the modulated signal.
44 subplot(2,2,3)
45 plot(t,u(1:length(t)))
46 xlabel('Time')
47 title('The modulated signal')
48 pause   % Press any key to see plots of the magnitude of the message and the
49 % modulated signal in the frequency domain.
50 subplot(2,1,1)
51 plot(f,abs(fftshift(M)))
52 xlabel('Frequency')
53 title('Spectrum of the message signal')
54 subplot(2,1,2)
55 plot(f,abs(fftshift(U)))
56 title('Spectrum of the modulated signal')
57 xlabel('Frequency')
58 pause  % Press a key to see a noise sample.
59 subplot(2,1,1)
60 plot(t,noise(1:length(t)))
61 title('Noise sample')
62 xlabel('Time')
63 pause  % Press a key to see the modulated signal and noise.
64 subplot(2,1,2)
65 plot(t,r(1:length(t)))
66 title('Signal and noise')
67 xlabel('Time')
68 pause  % Press a key to see the modulated signal and noise in freq. domain.
69 subplot(2,1,1)
70 plot(f,abs(fftshift(U)))
71 title('Signal spectrum')
72 xlabel('Frequency')
73 subplot(2,1,2)
74 plot(f,abs(fftshift(R)))
75 title('Signal and noise spectrum')
76 xlabel('Frequency')