Hioki 8847, Instruction Manual

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Appendix 5 FFT Definitions
A
17
Appendi
x
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 ampli-
tude of the linear spectrum is also reduced.
(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
N
1
N
2