Dual-mode dual-band ring resonator bandpass filter with transmission zeros

Dual-mode dual-band bandpass filters are developed based on a single ring resonator. The ring is composed of two half-wave stepped-impedance resonators with short-circuited ends, which are connected to ground vias by short high-impedance sections, providing inductive coupling between the resonating modes and hence bandwidth control. Experimental filters are fabricated and measured for validation. Skew-symmetric feed structure is employed in the first circuit for enhancing the frequency selectivity around the second band. Source-load coupling is incorporated with the second design to create transmission zeros on both sides of the two passbands, greatly improving the performance in transition bands. The measured results show good agreement with the simulation.