40
<Table V-2-1> Defined Parameters for FFT
Parameters
Definition
Equation
s
F
Sampling Frequency
User Definition
span
F
Useful Frequency Range
2.56
s
span
F
F
=
N
Buffer Size
User Definition
L
Display Line
1 or
2.56
2.56
N
N
L
L
=
+
=
f
Δ
Frequency Resolution
s
F
f
N
Δ =
t
Δ
Sampling Interval(sec)
1
s
t
F
Δ =
T
Recording Time
or Total Time of Saved Data in Buffer
T
N
t
= ⋅Δ
Freq
Freq
Buffer 1
Buffer 2
Buffer 3
FFT of Buffer 1
FFT of Buffer 2
[Fig. V-2-1] Instantaneous FFT results from each FFT calculation
As previously mentioned, the basic assumption of FFT analysis is that time signal has infinite
periodicity. But buffer size is so finite that energy leakage causes. So we call it “leakage error”. As
shown in [Fig. V-2-2], time signal with infinite periodicity generates just one peak in FFT results.
However, as shown in [Fig. V-2-3], time signal with finite buffer causes finite periodicity. If we
calculated FFT using this buffer, we got the broaden peak of FFT.
Summary of Contents for SLA-PA201
Page 64: ...64 Appendix 1 The Theory for calculating RMS by ISO2631 and ISO8041 ...
Page 67: ...67 Appendix 2 Frequency Response of Filter in the Vibration Level Meter ...
Page 75: ...75 Appendix 3 Theory of WBCombined Filter and Frequency Response ISO6954 Filter ...
Page 79: ...79 Fig 2 Lowpass Filter with 100Hz Cutoff Fig 3 a v transition Frequency Response ...
Page 88: ...88 Fig 12 Error between theoretical value and measured value ...
