Circuit Model Extraction of Parallel-Connected Dual-Passband Coupled-Resonator Filters

In this paper, an analytical coupling matrix extraction scheme is proposed for parallel-connected dual-passband coupled-resonator filters for the first time. Model-based vector fitting technique is adopted to fit the measured responses of the dual-passband filter (DPBF) with a set of rational functions, from which a transversal coupling matrix can be constructed. A novel coupling matrix transformation strategy is developed to transform the transversal coupling matrix to the one corresponding to the topology of a parallel-connected DPBF. A practical filter is used as a test example to demonstrate the extraction procedure. In diagnosing the coupling matrix of the test filter, two kinds of ambiguity problems are discussed in detail and are solved by a perturbation technique. This paper provides a general theoretic framework for computer-aided tuning of a parallel-connected dual-passband or multiple-passband filter.