Represent Analog Signals for Amplitude Modulation in MATLAB

Represent single-channel and multichannel analog signals for amplitude modulation in MATLAB®.

Define variables for the sampling rate, carrier frequency, and sampling times. Create a sinewave sampled at the sampling time increments. For sample time increments represented by t, the vector x results from sampling a sine wave 8000 times per second for 0.1 seconds. Use the ammod function to amplitude modulate the sine wave, and then plot the signals.

Fs = 8000;                 % Sampling rate in samples per second
Fc = 300;                  % Carrier frequency in Hz
t = [0:0.1*Fs]'/Fs;        % Sampling times for 0.1 seconds
x = sin(20*pi*t);          % Representation of the signal

y = ammod(x,Fc,Fs);        % Modulate x to produce y
figure;
subplot(2,1,1); plot(t,x); % Plot x on top
subplot(2,1,2); plot(t,y)  % Plot y below

Define a multichannel signal containing two sine waves. The first channel is a sinusoid with zero initial phase and the second channel is a sinusoid with an initial phase of pi/8. Use the ammod function to amplitude modulate the sine waves, and then plot the signals.

Fs = 8000;
t = [0:.1*Fs]'/Fs;
x = [sin(20*pi*t),sin(20*pi*t+pi/8)];

y = ammod(x,Fc,Fs);        % Modulate x to produce y
figure;
subplot(2,1,1); plot(t,x); % Plot x on top
subplot(2,1,2); plot(t,y)  % Plot y below

Phase-Modulate Analog Signals in AWGN Channel

Prepare to sample a signal for two seconds at a rate of 100 samples per second.

Define variables for the sampling rate, carrier frequency, sampling times, and phase deviation. Create a signal as the sum of a one-cycle and a two-cycle sinusoid. For sample time increments represented by t, the vector x results from summing two sine waves 100 times per second for 2 seconds. Use the pmmod function to phase-modulate the sine wave x. Add Gaussian white noise to the modulated signal, use the pmdemod function to recover the original signal, and then plot the orignal and recovered signals.

Fs = 100;           % Sampling rate
Fc = 10;            % Carrier frequency in Hz
t = [0:2*Fs+1]'/Fs; % Time points for sampling
phasedev = pi/2;    % Phase deviation for phase modulation

x = sin(2*pi*t) + sin(4*pi*t);

y = pmmod(x,Fc,Fs,phasedev);   % Modulate.
y = awgn(y,10,'measured',103); % Add noise.
z = pmdemod(y,Fc,Fs,phasedev); % Demodulate.
% Plot the original and recovered signals.
figure; plot(t,x,'k-',t,z,'g-');
legend('Original signal','Recovered signal');

Analog Modulation Filtering Examples