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    copied!<blockquote> <p>1) Why does the x-axis (frequency) end at 500? How do I know that there aren't more frequencies or are they just ignored?</p> </blockquote> <p>It ends at 500Hz because that is the <a href="http://en.wikipedia.org/wiki/Nyquist_frequency" rel="noreferrer">Nyquist frequency of the signal</a> when sampled at 1000Hz. Look at this line in the Mathworks example:</p> <pre><code>f = Fs/2*linspace(0,1,NFFT/2+1); </code></pre> <p>The frequency axis of the second plot goes from 0 to <strong>Fs/2, or half the sampling frequency.</strong> The Nyquist frequency is always half the sampling frequency, because above that, <a href="http://en.wikipedia.org/wiki/Aliasing#Sampling_sinusoidal_functions" rel="noreferrer">aliasing</a> occurs: <img src="https://i.stack.imgur.com/djBW3.png" alt="Aliasing illustration"> </p> <p>The signal would "fold" back on itself, and <em>appear</em> to be some frequency at or below 500Hz.</p> <blockquote> <p>2) How do I know the frequencies are between 0 and 500? Shouldn't the FFT tell me, in which limits the frequencies are? </p> </blockquote> <p>Due to "folding" described above (the Nyquist frequency is also commonly known as the "folding frequency"), it is physically impossible for frequencies above 500Hz to appear in the FFT; higher frequencies will "fold" back and appear as lower frequencies.</p> <blockquote> <p>Does the FFT only return the amplitude value without the frequency?</p> </blockquote> <p>Yes, the MATLAB FFT function only returns one vector of amplitudes. However, they map to the frequency points you pass to it.</p> <p>Let me know what needs clarification so I can help you further.</p>
 

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