Hioki MR8827, Инструкция по эксплуатации

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A
21
FFT Definitions
The following figure presents an example of spectral analysis by applying a window function to a time-
domain waveform.
Using the window function, discontinuous points on the time-domain waveform are eliminated, so the
wave shape approaches a line spectrum.
Spectrum
0 0.002 0.004 0.006 0.008 0.01
-0.1
0
0.1
Time [sec]
A
m
plit
ud
e
[V
]
0 10 20 30 40
-200
-100
0
Frequency [kHz]
M
agn
itud
e [dB]
Time-Domain Waveform
When a Blackman-Harris window function is applied to a time-domain
waveform (p. A18) in which the number of analysis points is not an integer
multiple of the input frequency
The following figure shows the time-domain waveform of the window function and its spectrum.
Each spectrum shows a large peak at a low frequency, and many smaller peaks at higher frequencies.
The largest peak is called the main lobe , and the smaller peaks are the side lobes .
The most accurate results of the FFT function are obtained when the width of the main lobe and the
amplitude of the side lobes are minimized, although both conditions cannot be satisfied at the same time.
Therefore, a window function with a wide main lobe is used when amplitude values are important, while
a window function with a small main lobe is used to observe fine spectral details, and a window function
with small side lobe amplitudes is used to exclude the effects of the surrounding spectrum.
However, the main lobe width is proportional to the width (1/W) of the window so increasing the number of
analysis points increases the frequency resolution.
Rectangular window
N-1 0
Amplitude
0
0 2 4 6 8 10
-80
-60
-40
-20
0
Frequency (1/W)
Gain [dB]
Time-Domain Waveform Spectrum
Hann window
N-1 0
Amplitude
0
0 2 4 6 8 10
-80
-60
-40
-20
0
Frequency (1/W)
Gain [dB]
Time-Domain Waveform Spectrum
Appx.