High-Selectivity Bandpass Filter With Wide Upper Stopband Using Harmonic Suppression Structure

A high-selectivity bandpass filter (BPF) with ultra-wide upper stopband is proposed in this paper. Four cross-coupled microstrip slow-wave open-loop resonators are utilized to implement a filtering response with high selectivity. To extend its stopband bandwidth, the folded stepped-impedance microstrip line with open stubs is introduced to suppress the harmonics. By cascading these two parts, a high selectivity BPF with wide stopband can be achieved. To verify the aforementioned mechanisms, the proposed BPF is implemented, fabricated, and measured. The attenuation slope of 168.6 dB/GHz and 265.3 dB/GHz in the lower and upper side of passbands is measured. Meanwhile, this filter can generate an ultra-wide upper stopband up to 25 GHz with rejection level larger than 24.2 dB.