Campbell CR9000, Инструкция по эксплуатации

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Section 3. CR9000 Measurement Details
3-24
WINDOWING
The FFT option allows radix-two (2
n
, where n = 5, 6, …16) transform lengths
ranging from 32 to 65,536 samples. Users can optionally apply a Hanning,
Hamming, Blackman-Harris, or one from a selection of Kaiser-Bessel beta
choices, window function to their time series before transforming them. The
beta (Kaiser-Bessel) allows the user to trade spectral leakage for spectral
resolution. Table 3.3-1 shows the maximum out of band leakage and the full
width, half of maximum (FWHM) spectral resolution monitoring a
monochromatic signal using four different betas. Chart 3.5-1 shows
graphically the bin resolution (or bin smearing effect) for no windowing, the
Hanning window and 4 Kaiser-Bessel betas.
Table 3.3-1. Spectral Leakage vs. Resolution
BETA
MAXIMUM LEAKAGE
(dB)
SPECTRAL RESOLUTION
(BINS)
8 -63 2.25
10 -74 2.50
12 -95 2.75
14 -110 3.00
CHART 3.5 COMPARISION OF SPECTRAL RESOLUTION FOR VARIOUS
WINDOWING FUNCTIONS
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
253 254 255 256 257 258 259 260 261
FREQUENCY BIN
PEAK SI
G
NAL
No Window
Hanning Window
Kaiser Window, Beta=8
Kaiser Window, Beta=10
Kaiser Window, Beta=12
Kaiser Window, Beta=14
Using a Kaiser-Bessler with a beta of around 12 results in a spectral leakage
that best matched the attenuation of the CR9052's anti-aliasing filters.
Although this spreads the FWHM of a single line source to 2.75 bins, this can
be compensated for by increasing the length (or number of bins) of the FFT
because the windowing spreads the signal across a finite number of bins, not
across an absolute frequency range.