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A method to obtain an accurate band-pass filter model from numerically computed or measured data is presented. A modified new version of the Cauchy technique that uses simultaneously both transmission (S21(f)) and reflection (S11(f)) parameters is applied in order to find a rational polynomial characterization of the filter. From this rational model the coupling structure can be extracted, revealing the differences with the designed one. An application example of a narrow-band and a wide-band model of a microstrip filter is presented.