Spectral leakage
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When taking the discrete Fourier transform (DFT) of a signal (e.g. to obtain the power spectral density), spectral leakage will occur if any of the input signal's frequency components lie between reference frequencies. This results in a blurring of those components.
Spectral leakage also occurs when the DFT is taken of a signal containing the remainder of a cycle. E.g. if a 3 Hertz sine wave is sampled for 0.9 seconds, a discontinuity results, which causes leakage.
E.g. someone can write an example showing how a DFT blurs a sinusoid depending on how the sinusoid's frequency compares to the reference frequencies. Also, one can discuss how window functions affect spectral leakage.