Campbell-scientific CR9000X Measurement and Control System Manual de usuario descargar pdf (Pagina 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

and the 65,536-point FFT gives an instantaneous dynamic range exceeding 140

dB (an amplitude ratio of 1x10

). Real-time digital temperature compensation

ensures gain accuracy (±0.03 percent of reading) and offset accuracy (±0.03

3-34

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Campbell-scientific CR9000X Measurement and Control System Manual de usuario Pagina 122

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