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April 5, 2020 12:57
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Plot periodic functions in Octave or Matlab with this script. It multiplies (modulates) the carrier and the msg signals, and plots the time domain and FFT reprisentation
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| % referenced gist https://gist.github.com/akiatoji/5649907 | |
| clear ; close all; clc | |
| fundPeriod = pi/1000; | |
| num_tsteps = 1000; | |
| num_periods = 4; | |
| tstep = num_periods * fundPeriod / num_tsteps; | |
| t = 0:tstep:(num_periods * fundPeriod); | |
| msg = 2*cos(400*t)+4*sin(500*t+pi/3); | |
| carrier = cos(8000*pi*t); | |
| modulatedMsg = msg.*carrier; | |
| f_s = num_tsteps / (fundPeriod*num_periods); | |
| n = 2**16; | |
| f = f_s*(-n/2:n/2-1)/n; | |
| subplot(2,2,3); | |
| plot(t, msg); | |
| title("message Time dom"); | |
| grid on; | |
| subplot(2,2,4); | |
| plot(t, carrier); | |
| title("carrier Time dom"); | |
| grid on; | |
| subplot(2,2,1); | |
| plot(t,modulatedMsg); | |
| title("modulated time domain"); | |
| grid on; | |
| y = abs(fft(modulatedMsg,n)); | |
| y = fftshift(y); | |
| subplot(2,2,2); | |
| stem(f, y); | |
| title("fast fourier"); | |
| grid on; | |
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Multiplying msg = 2cos(400t)+4sin(500t+pi/3); with carrier = cos(8000pit); will give us an AM modulated signal, and we see that it is a DSB-SC, as we have a double side band, and the amplitude at Fcarrier ( centre freq in the fft graph if you plotted it) is zero means that it is suppressed carrier, hence the SC.