  We discuss the problem of robust representations of stable and passive
transfer functions in particular coordinate systems, and focus in particular on
the so-called port-Hamiltonian representations. Such representations are
typically far from unique and the degrees of freedom are related to the
solution set of the so-called Kalman-Yakubovich-Popov linear matrix inequality
(LMI). In this paper we analyze robustness measures for the different possible
representations and relate it to quality functions defined in terms of the
eigenvalues of the matrix associated with the LMI. In particular, we look at
the analytic center of this LMI. From this, we then derive inequalities for the
passivity radius of the given model representation.
