Ultra-compact shorted ring resonator bandpass filter with wide upper stopband

An ultra-compact second-order bandpass filter with stopband extension is implemented with shorted ring resonators.In the design, the two resonances of the structure are separately controlled by electric and magnetic coupling between the rings. Two transmission zeros on both sides of the passband are created through the multipath effect between the I/O ports. Circuit miniaturization is accomplished by attaching an open low-impedance stub to each shorted ring. The upper stopband with a rejection better than 30 dB is extended to 4.32 times the passband frequency. The circuit area is only 8.8% of that of a conventional dual-mode ring resonator filter. Measured results show good agreement with the simulation.