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Explanation
IM WT3001E-51EN
11.5 Setting the Number of Computed Points and Time Window
• Selecting the Time Window
3.
Press the FFT Window soft key to display the FFT Window menu.
4.
Press the Rect, Hanning, or FlatTop soft key to select the time window.
• Setting the Number of Computed Points
You can select 200 k (200000 points) or 20 k (20000 points). The frequency resolution
varies depending on the number of computed points as follows:
Number of Computed Points
200 k
20 k
Note
If the number of computed points is set to 200 k, FFT takes approximately 10 s. The display
update interval is determined by the longer of the two values, the data update interval and (the
measurement period of the FFT + FFT computing time). Thus, it may take more than 10 s for
the display to be updated.
• Selecting the Time Window
Select from the three types listed below. For details on the time window, see section
11.1.
• Rect (Rectangular)
This window is effective on repetitive waveform of AC signals whose period is equal
to an integer multiple of the FFT measurement period. For example, the rectangular
window is effective on 50-Hz or 60-Hz signal with repetitive waveform.
• Hanning
This window is effective on waveforms that do not meet the conditions for the
rectangular window. The frequency resolution of the Hanning window is high
compared to the flattop window.
• FlatTop
This window is effective on waveforms that do not meet the conditions for the
rectangular window. The flattop window has a higher level of accuracy than the
Hanning window.
Frequency Resolution
Measurement Period for the FFT
1 Hz
1 s
10 Hz
100 ms
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Index
11-11
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