Construction of robust SPR functions

A new approach for the analysis of the strict positive real property of rational functions of the form H(s)=p(s)/q(s) is proposed, based on the interlacing properties of the roots of the even and odd parts of p(s) and q(s) over the imaginary axis. An algorithm to obtain p(s) such that p(s)/q(s) is SPR for a given Hurwitz q(s) is developed from the analysis of these properties. The extension of this result to the case when q(s) is an uncertain Hurwitz polynomial is also considered