nyqplot (System Identification Toolbox)

Plot Nyquist curve of frequency function.

nyqplot(g)
nyqplot([g1 g2 ... gn])
nyqplot(g,sd,mode)

Description

nyqplot is an alternative to bodeplot and ffplot to graph frequency functions in the freqfunq format. The Nyquist diagram of a frequency function is a graph of its imaginary part against its real part.

The argument g is the frequency function in question, given in the freqfunc format, typically as the output of th2ff, spa, or etfe. Several plots are obtained in the same diagram by simply listing the different frequency functions after each other. These need not be specified at the same frequencies (although they have to be of the same length).

If the frequency function(s) contains information about several different input-output pairs, the default is that the Nyquist plot for each pair is graphed separately. Pressing the Return key advances from one pair to the next. With mode = 'same' all plots corresponding to the same input are given in the same diagram.

If sd is given a value larger than 0, a confidence region around the nominal Nyquist plot is marked with dash-dotted lines. This region corresponds to sd standard deviations. (The information in g does not contain the correlation between the magnitude and phase estimates. The confidence region is therefore only approximate. It is computed by plotting the two curves

, where

is the standard deviation (magnitude and phase) of g.)

Examples

g = th2ff(thmod)
nyplot(g,3)