Campbell Scientific CR9000X Measurement and Control System - Manual (Page 275)

Section 7. Measurement Instructions
7-51
Parameter
& Data Type Enter FFTFILT PARAMETERS
FFTLen
Constant
The length of (number of points in) the time series snapshot on which to perform the FFT. If the
scan period equals FFTLen/FsampRate, then the consecutive time series
FFTLen
65536
32768
16384
8192
4096
2048
1024
512
256
128
64
32
snapshots processed into spectra are “seamless”. If the scan period is greater
than FFTLen/FsampRate, then the time series snapshots will have gaps
between them. A compile error occurs if the scan Period is less than
FFTLen/FsampRate
The FFT throughput for transforms of 2048 points or less is much higher than
the throughput for transforms longer than 2048 points. The CR9052DC can
produce spectra from seamless 50-kHz snapshots on six channels for FFTLen
equal to 2048 or less. The CR9052DC can produce spectra from seamless 50-
kHz snapshots on two channels for any FFTLen.
TSWindow
Constant
TSWindow designates whether the CR9052 should apply a window (also known as a taper, or
apodization) function to the time series snapshot before performing the FFT. Typical window
functions give more weight to the middle of the time series while tapering the ends to avoid
spectral leakage caused by a non-integral number of periods of a repetitive signal in the snapshot.
Numeric Code Window Function
0 None
1 Hanning
2 Hamming
3 Blackman
4nn Kaiser-Bessel nn: represents Beta (β) nn range: 5 - 16 (integer)
SpectOption Designates the output option for the computed spectrum.
Constant Numeric Code Spectra Result Maximum Spectrum Length
0 Real and Imaginary (FFTLen/2 + 1) pairs
1 Amplitude (FFTLen/2 + 1) values
2 Amplitude and Phase (FFTLen/2 + 1) pairs
3 Power Spectrum (FFTLen/2 + 1) values
4 Power Spectral Density
Function
(FFTLen/2 + 1) values
6 RMS Amplitude (FFTLen/2 + 1) values
7 DeciBels (FFTLen/2 + 1) values
CR9052 users may apply predefined spectral weighting functions to their output spectra with the
SpecOption. The CR9052 can implement A, B, and C spectral weighting for all spectral output
modes as defined in the IEC 60651 international standard. To select A-weighted spectra, add 10
to the SpecOption parameter. To select B-weighted spectra, add 20 to the SpecOption parameter.
To select C-weighted spectra, add 30 to the SpecOption parameter.Add 100 to the 100 to the
SpectOption codes above and the original time series data will be returned along with the
spectrum. The CR9000X places the time series data in the array Dest immediately following the
spectrum. When this option is enabled, Dest must be dimensioned large enough to hold this
additional time series data. If Reps is more than one, the CR9000X places the spectrum for the
first channel in Dest, followed by the time series for the first channel. Next, the CR9000X places
the spectrum for the second channel in Dest, followed by the time series for the second channel,
etc.