Abstract
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.
Highlights
Stopband suppression has always been a major concern for filter design
In [11], the multipath coupling technique was used to explain the principle of the transmission zeros (TZs) generated in the cascade triptych (CT) and cascade quadruple (CQ) topologies
A novel capacitive cross-coupling structure implemented in ceramic-filled cavity filters has been proposed to increase frequency selectivity
Summary
Stopband suppression has always been a major concern for filter design. In the past few decades, the cross-coupling technology has been popular due to its ability to obtain prescribed transmission zeros (TZs) close to the band edge for higher selectivity without increasing the number of resonant units [1]–[6]. A capacitive cross-coupling structure is proposed for ceramic-filled cavity filters. When the even mode has a higher resonant frequency than the odd mode, the coupling is capacitive In this case, the width of the metal strip is in the range of 0.023λ to 0.026λ, and the length of the metal strip is in the range of 0.084λ to 0.122λ. The simulation results tell us that only a longer metal strip may ensure capacitive coupling if the structure moves along the y-axis This conclusion coincides with the physical property of the sparse electric field distributions at the location of the coupling structure. Both the in-band response and stopband selectively should be considered
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