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 .
When an analog signal is
captured, it is sampled with fixed time intervals. Sampling fixed time
intervals can cause the actual waveform to get truncated at its start
and end. The results obtained can vary with the location of the sample
with respect to the wave form’s period. This results in discontinuities
in the continuous waveform.
The shape of the “leaky” spectrum depends on the amount of signal
truncation, and is generally unpredictable for real signals. A ‘window’
must be applied to the data to minimize signal ‘leakage’ effects.
Windowing is the equivalent of multiplying the signal sample by a window
function of the same length .
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