Design of a high-selectivity quad-band bandpass filter based on λ/4 resonators with alternative J/K inverters

Based on λ/4 resonators, the synthesis design method for a miniaturized high-selectivity microstrip quad-band bandpass filter (BPF) with alternative J/K inverters has been presented. Two dual-band λ/4 stepped impedance resonators (SIRs) are employed to generate the 1st and 4th passbands, while the 2nd and 3rd passbands are achieved by utilizing four coupled λ/4 uniform impedance resonators (UIRs). Moreover, the alternative J/K inverters form is proposed to create an additional cross coupling, resulting in one pair of transmission zeros (TZs) at each side of all the four passbands. To verify the validity of the proposed method, a quad-band BPF centered at 1.9, 4.1, 4.6 and 6.1 GHz with respectively fractional bandwidths of 13.7%, 5.1%, 5.9% and 7.6% has been designed and fabricated, whose measured results show consistent match with the simulated ones.