Bandpass Filters Using Single and Cascaded Novel Triple-Mode Ceramic Monoblocks

A novel compact triple-mode ceramic monoblock filter for 5G applications is proposed. The main resonator is a low-profile triple-mode ceramic cavity with silver coating and surface slots. By adding a source–load coupling cell to the single monoblock, the single monoblock filter with fully canonical transversal topology with three transmission zeros is achieved. This proposed filter with fully canonical transversal topology can achieve wideband, low insertion loss, and wide spurious-free window. To obtain sharper selectivity and higher out-of-band (OOB) rejections, a higher order filter is designed here by cascading two single monoblock filters with a microstrip line. Due to the very low profile and operating principle of the monoblock, there are no harmonics around <inline-formula> <tex-math notation="LaTeX">$1.2f_{\text {C}}$ </tex-math></inline-formula> (where <inline-formula> <tex-math notation="LaTeX">$f_{\text {C}}$ </tex-math></inline-formula> is the center frequency), which are inherent harmonics for triple-mode cubic dielectric cavity filters with three similar side lengths. The source–load coupling cell is also introduced to expand the spurious-free window by generating a transmission zero at higher harmonics. The proposed filters can achieve ultracompact size, no holes inside the blocks, and no rotated slots on silver surfaces, which are beneficial for system integration, mass production, and machining accuracy improvement. Two prototypes operating at 3.5 GHz (center frequency) have been fabricated. The measured and simulated results agree well with each other.