Analytical Circuit Model Extraction for Computer Aided Tuning of Microwave Filters and Diplexers
In this presentation, a novel rational function approximation method, named Model-based Vector Fitting (MVF) method will be introduced for analytically circuit model extraction for computer aided tuning of microwave filters and diplexers. The method incorporates the merits of conventional Cauchy method and vector fitting (VF) method. The MVF method allows pre-assigning of the orders of system zeros and poles. As the coupling topologies for channel filters determine the number of transmission zeros, the MVF method can accurately extract the characteristic functions of a diplexer without over-fitting and under-fitting problems. Having had the model determined, a transversal coupling matrix can be reconstructed from the measured S-parameters. Consequently, the transversal coupling matrix is transformed to the targeted coupling topology corresponding to the physical layout by a series of matrix similarity transformations. The extraction procedure is completely analytical and can be incorporated in a computer-aided tuning (CAT) program. Various practical diplexer cases incorporating a common resonator node junction and wired-T junction will be discussed and tuned with the help of the proposed circuit model extraction approach, showing a great effectiveness and the practical value of the proposed method. The model-based Vector Fitting method can also be applied to many other engineering problems, in which the orders of system zeros and poles need to be predefined.