Leakage
The FFT analyzer is a batch processing device; that is it samples the input signal for a specific time interval collecting the samples in a buffer, after which it performs the FFT calculation on that "batch" and displays the resulting spectrum
If a sinusoidal signal waveform is passing through zero level at the beginning and end of the time record, i.e., if the time record encompasses exactly an integral number of cycles of the waveform, the resulting FFT spectrum will consist of a single line with the correct amplitude and at the correct frequency. If, on the other hand, the signal level is not at zero at one or both ends of the time record, truncation of the waveform will occur, resulting in a discontinuity in the sampled signal. This discontinuity is not handled well by the FFT process, and the result is a smearing of the spectrum from a single line into adjacent lines. This is called "leakage"; it is as if the energy in the signal "leaks" from its proper location into the adjacent lines.
The shape of the "leaky" spectrum depends on the amount of signal truncation, and is generally unpredictable for real signals.
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