Data window

A phenomenon known as side-lobe leakage may lead to pronounced interac­tion between different frequencies with distorted spectra as a consequence. Applying a data window in the spectral analysis may reduce the influence of side-lobe leakage. The data window is a weight function used to taper a time series before the FFT analysis is initiated.   

Type

Three different data windows are available:

·         rectangular data window (i.e. without tapering)

·         Hanning data window (default)

·         Cosine squared

Overlap

The spectra (and response functions) are calculated using overlapping sub­series. The overlap controls the smoothing of spectra, but it also controls the weighting of the different parts of the input time series.

A value of zero corresponds to no overlap and with increasing overlap as the value tends towards unity. An overlap of 2/3 is used as default.

Number of subseries

Based on the specified FFT parameters the number of subseries can be cal­culated. It is almost always desirable to have a large number of subseries because it will reduce the variance of the spectral density and provide a smooth spectral estimate.

For non-overlapping time series the number of degrees of freedom for the spectral estimate is two times the number of subseries. Spectral estimates having 30-40 degrees of freedom is often used in practice.

Utilisation

Indicates the utilisation of the input data.