Iwatsu DS-8814, Operation Manual

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F-18
5. FFT Window Selection
When a FFT is performed, data within the computation range is processed as continuous
data. Therefore, unless an integral number of cycles of the input signal fit within the
computation range, the time axis waveform will be discontinuous and the spectrum
obtained by the FFT will spread out. This is called leakage.
Spectral leakage can be suppressed by use of an appropriate FFT window for the signal to
be analyzed.
(1) Burst waveform example (2) Repeating waveform example
As in (1), data is sampled within the computation range, and if points a and b are of a
continuous burst signal, spectral leakage will not occur. However, as in (2), if the front
and rear points c and d of the computation range are of a discontinuous signal, spectral
leakage will occur and accurate measurement cannot be performed.
If an FFT window is used, processing is implemented to reduce discontinuities at the front
and rear of the computation range, as in (2), and spectral leakage can be suppressed.
5.1 Rectangular (Rect)
A rectangular window is suitable for analysis of transient waveforms, as in the figure
below.
This window allows waveform elements near the beginning or end of a computation
range to be analyzed without attenuation.
YT waveform FFTwaveform