Coupled Resonator Bandpass Filters With Doubly coupled Input/Output

With the deployment of advanced wireless communication systems that employes massive MIMO antenna arrays, the demands for more practical coupling topologies for bandpass filters with asymmetric filter responses drastically increase. An attractive coupling topology needs to be compact in layout, with fewer/no diagonal couplings, compatible with dual-mode resonator realization and to generate more transmission zeros (TZs). Existing cascaded type of coupling topologies has found difficulties to fulfill the requirements. To this end, a new class of general coupling topologies with doubly coupled I/O ports is proposed in this paper. The new class coupling topologies exhibit two attractive advantages for asymmetric filtering responses: creating more transmission zeros (TZs) and requiring fewer or no diagonal couplings. To synthesize a doubly coupled I/O filter, a simple numerical synthesis method is also introduced. Several examples, including general cases of 8–4 filters with 4 specific practically useful coupling topologies and an 8–5 filter, are given to demonstrate the attractive features of the proposed doubly coupled I/O filters.