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We once had a homework way back when decided to share it. I edited this script which is in Matlab. I hope you like it. It simply is an analysis of AM and DSB-SC Modulation and Demodulation of a Periodic Square Wave.

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%Amplitude Modulation with a periodic sqare wave and its spectrum analysis %Show the time domain and frquency domain representation of DSB-AM and %DSB-SC modulations %By : John Roach – 6 March 2009 %visit my site at http://johnroach.info N = 1024; %N point FFT N>fc to avoid freq domain aliasing fs = 4096; % Sample frequency t = (0:N-1)/fs; fc = 600; %Carrier Frequency fm2 = 80; %Message Frequency Ec = 20; %Carrier Amplitude Em2 = 5; %Messagae Amplitude % Try changing the message and carrier amplitudes to see the effect in % DSB-AM modulation %———Double SideBand Full Carrier Modulation (DSB-FC(AM)) A = Ec + Em2*square(2*pi*fm2*t);%Envelope/eliminate the carrier amplitude m = A.*sin(2*pi*fc*t); %to convert DSB-AM to DSB-SC Mf = 2/N*abs(fft(m,N)); f = fs * (0 : N/2) / N;%Since the fft result is symmetrical, only the %positive half is sufficient for spectral representation close all; figure(’Name’,'Time/Fequency domain representations of DSB-AM signals’); subplot(2,1,1); %Time domain plot plot(t(1:N/2),m(1:N/2),t(1:N/2),A(1:N/2),’r',t(1:N/2),-A(1:N/2),’r'); title(’Time Domain Representation’); xlabel(’Time’); ylabel(’Modulated signal’); subplot(2,1,2); %Frequency Domain Plot plot(f(1:256),Mf(1:256)); title(’Frequency Domain Representation’); xlabel(’Frequency (Hz)’); ylabel(’Spectral Magnitude’); %———-Double SideBand Suppressed Carrier DSB-SC———- A = Em2*square(2*pi*fm2*t) ; %Envelope/eliminate the carrier amplitude m = A.*sin(2*pi*fc*t); %to convert DSB-AM to DSB-SC Mf = 2/N*abs(fft(m,N)); figure(’Name’,'Time/Fequency domain representations of DSB-SC signals’); subplot(2,1,1); %Time domain plot plot(t(1:N/2),m(1:N/2),t(1:N/2),A(1:N/2),’r',t(1:N/2),-A(1:N/2),’r'); title(’Time Domain Representation’); xlabel(’Time’); ylabel(’Modulated signal’); subplot(2,1,2); %Frequency Domain Plot plot(f(1:256),Mf(1:256)); title(’Frequency Domain Representation’); xlabel(’Frequency (Hz)’); ylabel(’Spectral Magnitude’); text(15,60,’Carrier’); %——————————————————————– |