how can i plot the amplitude spectrum of these signals
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i need to plot the amplitude spectrum of this signals
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/682248/image.png)
and
g(t)=![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/682253/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/682253/image.png)
the first fourier transform is: ![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/682258/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/682258/image.png)
and the second one is : ![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/682263/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/682263/image.png)
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Réponses (2)
Sulaymon Eshkabilov
le 12 Juil 2021
Fs = ... % Sampling freq
t = ... % Time
F = 0.25+cos(2*pi*50*t);
L = length(F);
N = 2^nextpow2(L);
Y = fft(F, N);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
f = Fs*(0:(L/2))/L;
plot(f,P1)
title('Single-Sided Amplitude Spectrum of S(t)')
xlabel('f (Hz)')
ylabel('|P1(f)|')
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Scott MacKenzie
le 12 Juil 2021
Modifié(e) : Scott MacKenzie
le 12 Juil 2021
Do you want the FFT of the combination of the 20 Hz and 50 Hz signals? If so, then this might be the sort of plot you are after:
duration = 1; % seconds
sRate = 8192; % default
sInterval = 1/sRate;
% vector for values of t at each sample point
t = 0:sInterval:duration;
n = length(t); % number of samples
% build the first wave vector, as per question (20 Hz)
y1 = 0.25 + sin(2*pi * 20 * t);
% build the second wave vector, as per question (50 Hz)
y2 = cos(2*pi * 50 * t);
% combine waveforms
y = y1 + y2;
% constrain samples to +/- 1
y = rescale(y, -1, 1);
% perform fast fourier transform
Y = fft(y);
% get amplitude vs. frequency spectrum
P2 = abs(Y/n);
P1 = P2(1:n/2+1);
P1(2:end-1) = 2*P1(2:end-1);
tiledlayout('flow');
% plot signals
nexttile;
plot(y);
set(gca,'ylim', [-1.2 1.2]);
title('Signal');
% plot fft of signals (up to 200 Hz only)
f = sRate*(0:(n/2))/n;
nexttile;
plot(f(1:200),P1(1:200));
title('Frequency Spectrum');
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/682413/image.jpeg)
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