A High-Selectivity D-Band Mixed-Mode Filter Based on the Coupled Overmode Cavities

A D-band low-loss mixed-mode filter with high selectivity is proposed and analyzed in this article. The transverse quintuplet/triple topology has been employed in the design with coupled overmode cavities using the TE $_{\mathrm {2N,1,0}}$ modes ( $N = 2,3,4, \ldots $ ) and TE $_{\mathrm {2N,1,0}}$ -like modes. Two transmission zeros (TZs) are generated in the upper and lower stopbands. The fractional bandwidth (FBW) and positions of the TZs can be flexibly designed by properly selecting the geometries of the proposed overmode cavity, which determines the frequencies of all resonating modes. To improve the frequency selectivity, multiple coupled overmode cavities, which have different spurious responses and TZs distributions, are cascaded for the designed filter. For demonstration purposes, two types of filter architectures are proposed for two D-band mixed-mode bandpass filters, respectively, which shows the design flexibility of the proposed method. The first filter is realized by cascading multiple overmode cavities through waveguide sections, and the second filter is realized by cascading overmode cavities through slots. Experiments are carried out to verify the second filter with TZs. The measured FBW3 dB is 17.18% from 128.11 to 152.19 GHz, the minimum insertion loss is around 0.33 dB, and the measured rectangle coefficient (BW40 dB/BW3 dB) is about 1.38. The measured results agree well with the designed ones, which verifies the proposed method. The proposed filter can be a good candidate for millimeter-wave and terahertz systems.