Spectral Models

The Spectral Model dialog is opened by chosing the corresponding item in the pop-up menu Estimate in the ident window.

This function estimates the frequency response of the Working Data set using spectral analysis. Open the Frequency resp model View to see the results.

The spectral model is also calculated if you press the hotkey s in the ident window.

There is a choice of methods and resolution parameter.

The frequency response is calculated for frequencies defined by the dialog created by selecting Frequency range... from the Frequency resp Options menu. The default is 128 frequency values, linearly spaced up to the Nyquist frequency. Note that the estimate is not recalculated if the choice of frequencies is changed at some later point.

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Resolution and Methods for Spectral Model

Resolution and Methods for Spectral Analysis

The trade-off between frequency resolution and uncertainty is controlled by the lag window M. It gives a frequency resolution of about pi/M rad/(sampling interval). The default choice of M - obtained by entering the empty matrix - gives a reasonable choice for not-so-resonant systems.

There is a choice between the Blackman-Tukey (SPA) window approach and a Smoothed Fourier Transform (ETFE). EFTE is a more efficient choice for high frequency resolution, but it only allows linearly spaced frequency values and does not compute confidence intervals or output disturbance spectra. EFTE is primarily intended for single or no input data, since the other inputs are treated as disturbances.

The algorithms are described under SPA and ETFE in the manual.

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(file spa.htm)