freqfunc (System Identification Toolbox)

Describe the frequency-function format.

help freqfunc

Description

Frequency functions are created by etfe, spa, and th2ff, and used by
bodeplot, ffplot, and nyqplot. The internal format of the freqfunc format is intended to be transparent. The basic way to display the information is to use the plot commands. You can also retrieve the information from the format by the function getff. This entry gives details of the internal representation, but this information is not necessary for normal use of the System Identification Toolbox.

The freqfunc format contains information about frequency values, amplitudes, and phases, as well as their standard deviations. These are all given as columns. The first row of a freqfunc matrix contains integers, that, in coded form, describe the significance of the column in question. The interpretation of these integers is as follows:

For transfer functions and spectra associated with output number 1:

n = 0 : The column is a spectrum.

n = 50: The column contains standard deviations of a spectrum.

n = 100: The column contains frequencies for the spectrum.

n = k, where k is value between 1 and 19: The column contains amplitude values for the transfer function associated with input number k.

n = k + 20: The column contains phase values (in degrees) for input number k.

n = k + 50: The column contains amplitude standard deviations for input number k.

n = k + 70: The column contains phase standard deviations for input number k.

n = k + 100: The column contains the frequency values for input number k.

For the same quantities associated with output number

, add

*1000 to the numbers above.

The specified frequencies are for a discrete-time model, which is by default, equally spaced from 0 (excluded) to

over 128 values. Here T is the sampling interval (default = 1). For a continuous-time model, the frequencies are 128 values, logarithmically spread over three decades, up to a decade over the underlying Nyquist frequency (see th2ff).

Examples

You can compute and graph the frequency functions at arbitrary frequency w (a row vector with an arbitrary number of elements) with

g = spa(z,M,w)
g = th2ff(th,ku,w)

The MATLAB function logspace is useful for creating such frequency vectors.