Positive-to-zero continuously tunable inter-resonator coupling structure for applications in filter array systems

This paper presents a novel positive-to-zero continuously tunable inter-resonator coupling structure for high Q substrate-integrated waveguide resonators. Two nearly nonmutually coupled distinct inter-resonator structures were integrated. One structure provides positive static inter-resonator coupling while the other composes a continuously tunable negative coupling. The negative coupling can be tuned to be large enough to cancel out the static positive coupling and create a near-zero inter-resonator coupling state. Using the proposed tunable structure, a second-order continuously tunable Butterworth filter has been designed and fabricated. The bandwidth of the fabricated filter can be tuned from 3% to 0% at 1.3 GHz. Similar tuning is possible for all frequencies within the frequency range 0.95 GHz ~ 1.6 GHz. The fabricated filter has been measured to have frequency tuning ratio of 1.7:1.