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Fourier Transform

Command: Math -> Fourier Transform...

The command Math/Fourier Transform... allows to perform a Fourier transformation of time series. It is assumed that the time series is stored in a particular column of the data matrix and that the data points are equidistant in time.

In order to select a variable you may either select a variable using the variable selection dialog (button ), or you may switch to the previous or next variable in the data matrix by using the arrow buttons ().

The length of the time window to be subjected to the Fourier transform has to be set in the "Sample Size" box. The length must be a power of 2. The first data point to be used for the analysis can be set by the control "First Data". The transformed time domain data are displayed in the uppert chart.

The resulting spectrum is shown in the bottom half of the window. The spectrum may be displayed using either a linear or a logarithmic amplitude axis. The weighting window of the FFT controls the form and width of the spectral lines, as well as the intensity of side lobes. Depending on the "Spectrum Type" selection box several different aspects of the spectrum are displayed (magnitude, power spectrum, phase, etc.).

In order to measure the energy in a particular range of the spectrum, you can specify an interval by dragging the crosshairs to inclose the interesting region of the spectrum. The RMS value is the square root of the sum of all lines of the power spectrum in the specified interval.

Last Update: 2012-Jul-25