Circuit Model Extraction of Coupled-Resonator Diplexers with Common Resonator from Two-Port S-Parameters

In this work, a circuit model extraction method for coupled-resonator diplexers with a common resonator junction is proposed and verified by tuning a high-order diplexer product. The entire extraction and tuning procedure is based on a two-port measurement with the other channel terminated by a matched load. Model-based vector fitting (MVF) is applied to fit the two-port measured data, from which the circuit elements associated to the common port can be extracted and removed by circuit transformation, leaving the measured channel as a standalone filter with the other channel as a dangling load. Then, the poles and residues of the two channel filters can be identified and classified according to magnitudes of residues, and the dangling channel is discarded. Thus, the coupling matrix of the channel filter under measurement can be obtained deterministically. The proposed two-port extraction method breaks down the tuning of a multi-port filtering network into a number of individual extraction and tuning of channel filters, laying a foundation for two-port circuit model extraction of a multiple channel multiplexer.