Specifications

Section 6. Data Table Declarations and Output Processing Instructions
FFT (SecArray, DataType, N, SampleInterval, Units, Option)
The FFT performs a Fast Fourier Transform on a time series of measurements
stored in an array. It can also perform an inverse FFT, generating a time series
from the results of an FFT. Depending on the output option chosen, the output
can be: 0) The real and imaginary parts of the FFT; 1) Amplitude spectrum.
2) Amplitude and Phase Spectrum; 3) Power Spectrum; 4) Power Spectral
Density (PSD); or 5) Inverse FFT.
Parameter
& Data Type
Enter
SrcArray
Variable
The name of the Variable array that contains the input data for the FFT.
DataType
A code to select the data storage format.
Constant
Alpha
Code
Numeric Code Data Format
IEEE4 24 IEEE 4 byte floating point
FP2 7 Campbell Scientific 2 byte floating point
N
Constant
Number of points in the original time series. The number of points must be a power of 2
(i.e., 512, 1024, 2048, etc.).
SampleInterval
Constant
The sampling interval of the time series.
Units
The units for Tau.
Constant
Alpha Code Units
USEC microseconds
MSEC milliseconds
SEC seconds
MIN
HR
Minutes
Hours
Option
A code to indicate what values to calculate and output.
Constant
Code Result
0
1
2
3
4
5
FFT. The output is N/2 complex data points, i.e., the real and
imaginary parts of the FFT. The first pair is the DC component and
the Niquist component. This first pair is an exception because the DC
and Niquist components have no imaginary part.
Amplitude spectrum. The output is N/2 magnitudes. With Acos(wt);
A is magnitude.
Amplitude and Phase Spectrum. The output is N/2 pairs of magnitude
and phase; with Acos(wt - φ); A is amplitude, φ is phase (-π,π).
Power Spectrum. The output is N/2 values normalized to give a power
spectrum. With Acos(wt - φ), the power is A
2
/ 2. The summation of
the N/2 values yields the total power in the time series signal.
Power Spectral Density (PSD). The output is N/2 values normalized
to give a power spectral density (power per herz). The Power
Spectrum multiplied by T = N*tau yields the PSD. The integral of the
PSD over a given bandwidth yields the total power in that band. Note
that the bandwidth of each value is 1/T hertz.
Inverse FFT. The input is N/2 complex numbers, organized as in the
output of option 0, which is assumed to be the transform of some real
time series. The output is the time series whose FFT would result in
the input array.
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