Campbell Scientific CR9000X Measurement and Control System - Manual (Page 122)

Section 3. CR9000X Measurement Details
CHART 3.3-4 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 SIGNAL
No Window
Hanning Window
Kaiser Window, Beta=8
Kaiser Window, Beta=10
Kaiser Window, Beta=12
Kaiser Window, Beta=14
Hanning
Kaiser, Beta = 14
No Windowing
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.
SPECTRAL OUTPUT
The CR9052 offers a variety of spectrum normalizations, including real and
imaginary, amplitude and phase, power, power spectral density (PSD), and
decibels (dB). In addition, the CR9052 can combine adjacent spectral bins into
a single bin to decrease the size of the final spectrum. A built-in function
selects an exponentially increasing spectral bin width to give 1/n octave
analyses, where n can vary from 1 to 12. A single programming step with
either the CRBasic programming language or the CR9000X program generator
configures the FFT spectrum analyzer options.
The module has superior noise performance, with an input-referred noise of
eight nano-volts per root hertz (8 nV/Hz
1/2 ) for the ± 20 mV input range. On
the ± 20 mV input range, the total noise for a 20 kHz bandwidth is less than
1.4 uV, and for a 1 Hz bandwidth, 250 nV. The programmable anti-alias filter
allows users to trade bandwidth for noise, or vice versa. The DSP's floating-
point numeric implementation of the FIR anti-alias filters and Fourier
transforms preserve this low-noise performance. A 2048-point FFT gives an
instantaneous dynamic range exceeding 126 dB (an amplitude ratio of 2x10
6
)
,
and the 65,536-point FFT gives an instantaneous dynamic range exceeding 140
dB (an amplitude ratio of 1x10
7
)
. Real-time digital temperature compensation
ensures gain accuracy (±0.03 percent of reading) and offset accuracy (±0.03
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