Measurement-computing eZ-PostView rev.2.0 Manuel d'utilisateur télécharger pdf (Page 104)

4-26 Edit Menu 969795 eZ-Analyst

Edit Menu

> Configuration > FFT Setup Tab

Applies to eZ-Analyst & eZ-FrequencyView

FFT Setup Tab

Edit Menu > Configuration > FFT Setup Tab: FFT Window Functions / Response Channels

The FFT (Response) window is a time-domain, weighting window. A response window is

usually applied to data to reduce FFT leakage errors. FFT theory assumes that the signal

being analyzed is periodic in the data acquisition block. When this is not the case, energy

from a signal at a specified frequency can leak into nearby spectral bins causing spectral

amplitude inaccuracies. Applying a windowing function controls, but doesn’t completely

eliminate, the error by multiplying each data frame by a suitable time-domain weighting

window. This calculation reduces the amplitude/magnitude of the data near the ends of each

data frame prior to performing the FFT and forces the data to be nearly periodic in the

window, thus reducing leakage errors. Response window options are as follows:

FFT Window Functions, Response Channels Pull-down List

None:

No weighting window is applied.

Hanning:

The Hanning window is typically used to analyze continuous signals. It offers

a reasonable trade-off of frequency accuracy versus amplitude accuracy.

FlatTop:

Compared to the very similar 4-term “Max Flat Top,” this window also has a

very low peak amplitude error, and its frequency resolution is somewhat

better. Its side lobes are considerably higher. Its effective noise bandwidth is

still almost twice that of the Hanning window, therefore this window is used

mainly to measure accurate peak amplitudes of discrete spectral components

that are known to be separated by several spectral lines.

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Measurement-computing eZ-PostView rev.2.0 Manuel d'utilisateur Page 104