Substrate integrated waveguide quasi-elliptic filters with controllable electric and magnetic mixed coupling

This paper proposes a type of quasi-elliptic filter with controllable electric and magnetic mixed coupling based on substrate integrated waveguide cavity resonators using two-layer printed circuit board process. For the second-order mixed coupling filter, an embedded short-ended strip line is combined with a conventional inductive window between two cavities to create the mixed coupling, in which the electric and magnetic coupling can be separately controlled by adjusting the width of the strip line and the inductive window, and a controllable transmission zero can be produced below or above the passband. For local multipoint distribution service application, three directly coupled quasi-elliptic filters and two cross-coupled quasi-elliptic filters are designed, simulated, and fabricated using the mixed electric and magnetic coupling second-order filtering units at the Ka-band. For the N-order filter, a maximum of N-1 finite transmission zeros can be realized. The proposed filters show the advantages of good selectivity with controlled transmission zeros, wide upper stopband, and easy integration with other planar circuits. Good agreements have been observed between the measured results and the simulated ones.