A Compact Wideband BandPass Filter Based on Parallel-Coupled Stub-Loaded Resonator

In this study, a compact high selectivity wideband bandpass filter (BPF) based on parallel-coupled stub-loaded resonator is proposed. The operating bandwidth and return loss can be adjusted by changing the coupling coefficient of the parallel-coupled line and the characteristic impedance of the central loaded stub. Good frequency selectivity and harmonic suppression are realized due to four transmission zeroes are distributed in the lower and upper side of the passband, the transmission zeroes are fixed once the center operating frequency is determined. To validate the proposed topology, a compact BPF center at 3 GHz with a measured rejection level higher than 31 dB in the lower stopband and larger than 28 dB till to 7.47 GHz (2.52 f(0)) in the upper stopband is designed and fabircated. Good agreement among the calculated, simulated and measured is observed.