Stability

The relevant notion of stability is Bounded-Input Bounded-Output (BIBO) stability. In general, for any set of frequency-domain data, it is possible to generate both stable and unstable state-space models which fit the data perfectly.

but unstable models are useless in a time-domain simulation because they result in diverging time-domain signals. In the frequency domain, the condition of stability for a state-space model is that the poles of the system, which are the eigenvalues of A, must have negative real parts. All state-space models generated by Ansys’s tools are stable.