Wrong amplitude values after the FFT

At the plot of the code that written below, i should see amplitude between values of 75 to 200 uV but there is something strange in my plot, can you help me and advise, what wrong with my code and how can i correct that ?

Thank you all

Here is the code :

clear all;
close all;
Fs = 200
t= 0:1/Fs:180
y_in=zeros (1, length(t));
  for  i = 1:18
F = randi ([4 7], 1);% frequency
A = randi ([75 200],1);% amplitude A=75~200 uV.
y_tmp =A*sin (2*pi*F*t);
 y_in=y_in+ y_tmp;
L=length (y_in);
NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y_new = fft(y_in,NFFT)/L;
f_new = Fs/2*linspace(0,1,NFFT/2+1);

plot(f_new,2*abs(Y_new(1:NFFT/2+1))) ;
title('Single-Sided Amplitude Spectrum of y_in(t)')
xlabel('Frequency (Hz)')


answered 6 years ago Webfoot Witch Hat #1

Since you're looping 18 times and use

F = randi([4 7],1);

some of the frequencies that build up the final signal occurs multiple times, hence their amplitudes are added to each other.

What you're seeing after the fft in your plot is therefore the sum of the amplitudes for the frequencies

F = 4, 5, 6, 7 Hz

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