A Novel Capacitive Cross-Coupling Structure for Ceramic-Filled Cavity Filters

Since the probe coupling for cavity filters is unsuitable for ceramic-filled cavity filters, a novel capacitive cross-coupling structure is proposed to improve stopband suppression by introducing transmission zeros. This structure is implemented by first etching a coupling window on the silver-plated layer on the ceramic surface between two resonant cavities and then placing an isolated metal strip in the window. This structure enjoys high design freedom. Thus, the coupling coefficient between the two cavities is readily adjusted. Moreover, the structure has the advantages of a compact size and simple fabrication and tuning processes. For validation, a ten-pole ceramic-filled cavity filter for 5G applications is designed and evaluated. The proposed cross-coupling technology is used to introduce a pair of symmetrical transmission zeros at the band skirts to increase the frequency selectivity. The measured results are consistent with the simulated ones.