Computer‐aided diagnosis of lossy microwave coupled resonators filters

A method is presented for extracting the coupling matrix (CM) and the unloaded Q from the measured (or electromagnetic simulated) scattering parameters of a lossy coupled resonators bandpass filter. The method can be used for computer‐aided tuning of a microwave filter. The method consists of two elements: 1) a three‐parameter optimization method is proposed to obtain the unloaded Q (assuming all the resonators with the same unloaded Q) and to remove the phase shift of the measured S ‐parameters caused by the phase loading and the transmission lines at the input/output ports of a filter; 2) the Cauchy method is used for determining characteristic polynomial models of the S ‐parameters of a microwave filter in the normalized low‐pass frequency domain. Once the characteristic polynomials of the S ‐parameters without phase‐shift effects are determined, the CM of a filter with a given topology can be extracted using well‐established techniques. Three diagnosis examples illustrate the validity of the proposed method. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.