A Dual-Mode Building Block Based on Frequency-Dependent Coupling for Bandpass Filter Synthesis Design

This article presents a method for designing bandpass filters by introducing frequency-dependent coupling (FDC) into dual-mode resonant cavities. In a single cavity, an electric coupling post and a magnetic coupling post are employed to simultaneously couple the TM120 (TE120) and TM210 (TE210) modes. The utilization of a mixed coupling structure enables the establishment of an FDC characteristic, thereby facilitating the introduction of a controllable transmission zero (TZ) near the passband. The coupling coefficient and the TZ position can be flexibly controlled by adjusting the physical parameters of the two posts. The proposed coupling structure is advantageous because a very wide tuning range of the coupling coefficient can be achieved. In this article, the proposed dual-mode resonator with FDC is considered as a building block to realize various high-selectivity filters. To validate the concept, the building block is first used as a second-order filter to be analyzed in detail. Two building blocks are cascaded to configure inline fourth-order filters. Finally, to verify the flexibility in applying different topologies, a traditional dual-mode cavity is used as a transversal unit to cascade with two proposed building blocks, resulting in the design of a sixth-order filter. Furthermore, fourth- and sixth-order filters at 5 GHz with a bandwidth of 100 MHz are fabricated as a demonstration. The measured results are in good agreement with the results of the simulations.