Single-mode ring resonator for bandpass filter application

A novel ring resonator which is fed along one of its quarter-wavelength sides is presented. The resonator´s response presents a single-mode resonant frequency with a transmission zero at both sides of its passband. Two such resonators are proposed to be combined in series to result in a 3^rd order bandpass response with a good out-of-band rejection level. The resonator and filter are simulated and fabricated on FR4 microstrip substrate. All the simulation and measurement results are presented in this paper to show its feasibility.