Vector Fitting is a robust numerical method for rational approximation in the frequency domain. It permits to identify state space models directly from measured or computed frequency responses, both for single or multiple input/output systems. The resulting approximation has guaranteed stable poles that are real or come in complex conjugate pairs.

This web site gives information about the method and provides a downloadable Matlab implementation. Also is provided a procedure for passivity enforcement.

Vector Fitting is best described in:
B. Gustavsen and A. Semlyen, "Rational approximation of frequency domain responses by vector fitting",
IEEE Trans. Power Delivery, vol. 14, no. 3, pp. 1052-1061, July 1999.  PDF

Last updated: 2009-08-27