Broadband implementation of tunable, substrate-integrated, evanescent-mode, cavity bandpass filters

A novel structure for implementing broadband external and internal coupling in heavily loaded, substrate integrated, evanescent-mode, cavity filters is presented. The proposed structure is based on adding electric field coupling to the magnetic field iris coupling, which is commonly used for such cavities. It is shown that fractional bandwidth on the order of 40% can be achieved using this new structure. As an illustration, a second order Butterworth filter is designed using the presented coupling method and then fabricated inside a two-layer RF circuit board. The measured and simulated filter results are shown to be in good agreement, with measured insertion loss of around 0.4 dB including connector losses. Finally, the filter is continuously tuned from 2910 MHz to 3795 MHz.