Channel Filter Extraction for Coupled-Resonator Diplexers

This article presents a circuit model extraction method for the coupled-resonator diplexer with a common resonator junction from S-parameters. A new rational fitting model is applied to the circuit model extraction with the aid of vector fitting. The characteristic admittance functions of each channel filter and resonator junction can be extracted separately. With the extracted characteristic functions, the circuit model construction of each channel filter can be implemented independently and simultaneously. The proposed method can naturally guarantee no interchannel parasitic coupling in the extracted circuit model. Then, the topological matching between the extracted circuit model and physical realization is enhanced. The complete circuit model extraction procedure is analytical and can be applied to computer-aided design and tuning of diplexers. For verification purposes, circuit model extraction examples of an electromagnetic (EM) simulated diplexer and a fabricated diplexer are demonstrated, respectively.