Campbell CR800 Dokumentacja Pobierz pdf (Strona 214)

Section 8. Processing and Math Instructions

Spectrum; 3) Power Spectrum; 4) Power Spectral Density (PSD); or 5) Inverse

FFT.

The difference between the FFT instruction (Section 6) and FFTSpa is that

FFT is an output instruction that stores the results in a data table and FFTSpa

stores its results in an array.

Parameter

& Data Type

Enter

Dest

Array

The array in which to store the results of FFT.

Source

Variable

The name of the Variable array that contains the input data for the FFT.

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.).

Tau

Constant

The sampling interval of the time series.

Units

The units for Tau.

Constant

Alpha

Code

Numeric

Code

Units

USEC 0 microseconds

MSEC 1 milliseconds

SEC 2 seconds

MIN 3 minutes

Options

A code to indicate what values to calculate and output.

Constant

Code Result

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. 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 herz.

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.

T = N*tau: the length, in seconds, of the time series.

Processing field: “FFT,N,tau,option”. Tick marks on the x axis are 1/(N*tau)

Herz. N/2 values, or pairs of values, are output, depending upon the option

code.

8-12

1 2 ... 209 210 211 212 213 214 215 216 217 218 219 ... 323 324

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Campbell CR800 Dokumentacja Strona 214

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