Microstrip ring resonator filter with inductively-compensated parallel-coupled feed and stepped-impedance design

A dual-mode ring resonator filter design technique is reported that uses a parallel-coupled feed structure and incorporates a stepped- impedance lowpass filter in the ring. The structure yields a compact bandpass filter element having two poles and two zeroes with improved harmonic suppression. The proposed topology is demonstrated with a design operating at 2.45 GHz with 5% bandwidth. More than 18 dB of spurious suppression from 2.8 to 8 GHz is demonstrated in the experimental results.