High-Selectivity Tunable Filters With Dual-Mode SIW Resonators in an L-Shaped Coupling Scheme

This article reports the design of high-selectivity tunable bandpass filters with constant absolute bandwidth (CABW) using dual-mode (DM) substrate-integrated-waveguide (SIW) resonators. The even and odd modes are coupled to the source and load using an L-shaped coupling scheme instead of the conventional doublet scheme, and thus, each mode produces either one reflection zero or one transmission zero (TZ). Each resonator has one contactless tuner to change the frequencies of both modes simultaneously, and a controllable separation between the two modes can be obtained over the whole tuning range (TR). The proposed resonator is utilized as the building block in four prototype filters with different TZ locations. The first filter is in a principle back-to-back topology, and it distributes TZs on both sides of the passband and has a CABW across the entire TR. The rest three further explore techniques to improve the out-of-band selectivity, i.e., introducing source/load coupling to increase the reject level and relocating the TZs on one side of the passband to build a spectrum with high rejection. All filters are based on two DM SIW resonators with only two tuners. Compared to state-of-the-art works, the proposed filters offer a wide constant-BW TR, low insertion loss, high selectivity, and good nonlinear performance.