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I am very new to this things. Sorry for probably stupid question. I don't understand what units and meaning have the values on Y-axis of Fourier Transform graph? On X-axis it is Frequency (Hz). Pretty clear. Can't find the meaning of Y.

Here is the code in MATLAB I used to produce the graph, and the image of the output:

A = 5;
t = 0:1/1000:1;
f1 = 7;
s = A * sin(2 * pi * fi * t);
F = fft(s);
plot(abs(F)), (s);

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Are we talking about the discrete Fourier transform (DFT) or the continous Fourier transform? Note that the unit of the x-axis of the DFT (or FFT) is not Hz, it's the normalized frequency. – Deve Nov 7 '12 at 9:58
up vote 1 down vote accepted

The Y-axis units are the same as the Y-axis units you would have if you plotted the time-domain signal. For instance, if the samples that you input into the FFT are the signal voltage, then the FFT units are also voltage. To get energy units you have to square the absolute value of the FFT.

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