Why are the height of the peaks different in the “FFT for Spectral Analysis” demo in MATLAB 7.11 (R2010b)?
I am running the fftdemo.m file that ships with MATLAB. The resulting power spectral density plot shows two peaks with different amplitudes. Because the original two sine waves that were generated have the same amplitude, I expect that the peaks of the PSD plot have the same amplitude as well.
ANSWER
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The two sine waves do have the same power, but it is not immediately evident just by looking at plot of Pyy. There are two reasons why. The first reason is because of the random noise that is being added. If you were to rerun the code several times, you would see the height and width of the two peaks change. This is because the noise that is being generated will be different on successive runs.
Now let’s get the noise out of the picture. Try running this code:
t = 0:.001:.25;x = sin(2*pi*50*t) + sin(2*pi*120*t);X = fft(x,251);Pxx = X.*conj(X)/251;f = 1000/251*(0:127);plot(f,Pxx(1:128))
You will see two peaks of different height. Note that the power is not determined by the height of the peak, but by the integral of the peak. If you execute:
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