Hioki MR8740, Инструкция по эксплуатации

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Appendix 4 FFT Definitions
A
19
Appe
ndix
Averaging_____________________________________________________
With the FFT function, averaging is performed according to the following analyti-
cal expressions. Averaging in the time domain produces meaningless data if per-
formed with inconsistent trigger criteria.
1. Simple Averaging (Time and Frequency Domains)
Sequences of acquired data are summed and divided by the number of acquisi-
tions.
n : count of measurements to average
A
n
:
averaging results of
n
counts
Z
n
: measurement data of n counts
2. Exponential Averaging (Time and Frequency Domains)
Before averaging, newer data is given exponentially greater significance than
older data.
N : Specified number of counts to average
n : count of measurements to average
A
n
: averaging results of n counts
Z
n
: measurement data of n counts
Overall Value __________________________________________________
The overall value is the sum of the power spectrum at each frequency. This
value is equal to the positive sum of the squares of the (RMS) input signals,
except when frequency averaging is performed. The FFT function of this instru-
ment calculates and displays the RMS values for stored waveforms and the
overall value from the sum of the power spectrum for the frequency domain.
P
i
: power spectrum of value
i
Total harmonic distortion (THD) __________________________________
Total harmonic distortion (THD) indicates the proportion of the higher harmonics
to the fundamental harmonic
This means that the larger the value, the more distorted the waveform.
(11)
n
Z A n
A
n n
n
+ −
= − 1
) 1 (
(12)
N
Z A N
A
n n
n
+ −
=
−
1
) 1 (
(13)
∑
=
=
0
) (
i
i
P all Over
THD
Σ
f
n
( )
2
f
0
( )
2
--------------- 100 ×
=
f
0
= fundamental wave
f
n
=
n
next higher harmonic
[%]