Triple-Mode Ring Resonator Microstrip Bandpass Filter with Enhanced Rejection

This paper presents a novel triple-mode ring resonator wideband bandpass filter with enhanced wideband rejection characteristics. The proposed ring resonator is half replaced by a pair of λ_g/4 open-ended coupled lines connected in cascade with λ_g/4 shunt open stub. The stepped impendence resonator can enhance the rejection of higher order modes. It has three resonance frequencies in the passband and exhibits attenuation poles close to the edges of the passband. The filter is designed at center frequency of 1090MHz and simulated by the commercial software IE3d. The measurement results are in good agreement with the simulations.