High selectivity wideband bandpass filter based on stepped impedance open stubs loaded ring resonator

Abstract A high selectivity wideband bandpass filter (BPF) is proposed based on a stepped impedance open-stub loaded ring resonator. The resonant modes can be adjusted by the parameters of the stepped impedance stubs and eventually to tune the bandwidth conveniently. The even resonant modes can be tuned lower than the first odd resonant mode that is excited by the half wavelength resonator. Due to the analysis of the electric field distributions on the resonator, three adjacent resonant modes can be employed to form a wide bandwidth by electric coupling between input/output feeding line and the resonator. In addition, by introducing two more resonant modes in the passband using the parallel-coupled feeding open stubs, the selectivity is further improved. The related coupling topology is theoretically analyzed and experimentally verified. As the design example, a wideband BPF centered at 3.24 GHz with a 3-dB fractional bandwidth of 58.3% is fabricated and measured. The experimental results fit the simulations well.