Design of a wideband bandpass filter using microstrip parallel-coupled dual-mode ring resonator

In this paper, a micostrip parallel-coupled dual-mode ring resonator is analyzed based on its equivalent transmission line circuit and the even- and odd-mode method. Closed-form formulas relating the transmission zeros and 3dB fractional bandwidth of the resonator to the circuit parameters are derived, and are verified effective by using a circuit simulator and an electromagnetic simulator. A bandpass filter is designed by using this type of dual-mode resonator, and it shows a wide passband with a 3dB fractional bandwidth of 30%, low reflection loss in the passband, and very sharp passband skirt property.