Hioki MR8827, Instruction Manual

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A
15
FFT Definitions
■ Number of Analysis Points
The FFT functions of this instrument can perform frequency analysis of time-domain waveforms consisting
of 1000, 2000, 5000, or 10,000 points. However, when the following conditions are satisfied, previously
analyzed data can be reanalyzed with a different number of analysis points.
A. When measurements are made with the averaging function disabled (Off)
B. When measurements are made with the averaging function enabled for time-domain averaging (simple
or exponential).
When the number of analysis points at measurement time is
N
1
and the number of analysis points is
changed to N
2
after measurement, the instrument performs as follows.
(1) When N
1
< N
2
• Because not enough data has been collected, zero is inserted for time after the end of the measured
waveform.
• The window function applies only to the N
1
segment.
• Frequency resolution is increased. For example, if N
1
= 1000 and N
2
= 2000, frequency resolution is
doubled.
• The average energy of the time-domain waveform is reduced, so the amplitude of the linear spectrum is
also reduced.
N
1
N
2
(2) When
N
1
>
N
2
• The specified (
N
2
) segment is extracted from the head of the (
N
1
) data.
• The window function applies only to the N
2
segment.
• Frequency resolution is decreased. For example, if
N
1
= 2000 and
N
2
= 1000, frequency resolution is
halved.
• The average energy of the time-domain waveform is unchanged, so the amplitude of the linear
spectrum is not significantly affected.
N
1
N
2
Appx.