Campbell CR9000, Instruction Manual

Page: 150 / 218

Section 7. Measurement Instructions
7-36
Parameter
& Data Type
Enter
FFTFILT INSTRUCTION PARAMETERS
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
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.
FRef
Constant
Reference Frequency for Logarithmic rebinning. Set to 0 for linear or no rebinning.
SBin
Constant
For linear rebinning: the number of adjacent spectral bins to combine. For logarithmic
rebinning: the number of bins per octave in the rebinned spectrum. Set to 0 or 1 for no
rebinning. The DC component, bin 0, is left alone and not combined with other bins. Bin
combination starts with the first AC component. Combining bins is not allowed for the Real
and Imaginary or Amplitude and Phase spectral options.
ILow, IHigh
Constants
ILow and IHigh make it possible to return a subset of the spectrum that results from the
Spectral option and bin combining specified by the previous parameters. I is the bin number.
ILow is the number of first bin to return, IHigh the number of the last bin to return. To get all
the components set ILow equal to the lowest bin number and IHigh to the maximum bin
number. With linear spectral bins (Fref = 0), the lowest bin number is 0 and the highest bin
number is the integer portion of FFTLen/(2*Sbin). See the text for details and logarithmic
rebinning (Fref
≠
0).