How to Write a matlab code to plot the amplitude modulated signal in time domain.

How to Write a matlab code to plot the amplitude modulated signal in time domain. Modulating signal = cos2π(300t) Carrier signal = cos2π(5000t)

Answer 1

[Scott MacKenzie]

Here's one way to do this. I'm showing two graphs, one with 5 Hz amplitude modulation, one with 5 kHz amplitude modulation. I've also passed the modulated signal into the sound function, so you can hear the result as well. At 5 Hz, the effect in music is known as tremolo.

duration = 3; % seconds
samplingRate = 8192; % per second
samplingInterval = 1 / samplingRate;

t = 1:samplingInterval:duration;
y1 = cos(2*pi*300*t); % signal
y2 = cos(2*pi*5*t);   % modulating signal (frequency = 5 Hz, adjust as desired)
y2 = rescale(y2,0.25,1); % modulation amplitude min/max factor

y = y1 .* y2; % create amplitude-modulated signal

% plot about 1 second of the signal (to show amplitude modulation)
plot(y(1:9000))
set(gca,'ylim', [-1.2, 1.2]);

% play it
sound(y, samplingRate); % NOTE: sRate not needed if 8192 (default)

5 Hz modulation:

5 kHz modulation:

[Yash Yadav]

Nice understandable

[Md. Sakib-Ul-Islam]

what if i want to see the frequency spectrum of this modulated wave? can i get a code for that?

Answer 2

[MD Rasel Basunia]

%% taking input of different parameter
Ac=1;%carrier Amplitude
Mu=0.9 % modulation index (let)

fm=300 % message signal frequency
fc=5000 % carrier frequency
fs=100*fm % sampling frequency with oversampling factor 100
%% setting proper time intervals
%t = [0:1e-8:1e-2];
t=[0:1/fs:1e-2];
%% compute the AM signal
x_am_1 = Ac*(1+Mu*cos(2*pi*fm*t)).*cos(2*pi*fc*t);%Am equation

%% compute envelop 
x_envelop_1 = abs(Ac*(1+Mu*cos(2*pi*fm*t)));% equation of envelop

%% Graph of Am and its envelop
subplot(211)
plot(t,x_am_1,t,x_envelop_1,'r','linewidth',1.5);grid on;
xlabel('Time(sec)');ylabel('Amplitude');legend('AM signal','Envelop');
title('Single sided envelop of AM with modulation index Mu=0.9');

%% Double side envelop
subplot(212)
plot(t,x_am_1,'k',t,x_envelop_1,'r',t,-x_envelop_1,'b','linewidth',1.5);grid on;
xlabel(' time ');ylabel('Amplitude')
legend('AM signal','Double sided envelop');
title('Double sided envelop of AM with Mu = 0.9');

% you can change modulation index in the above code.
% output:

Answer 3

[safuan khatimi]

how to find modulation index?