A dual-band microstrip filter topology based on quarter-wavelength coupled-lines inter-connections

A dual-band bandpass filter topology based on coupled-lines is presented. In the topology, two identical coupledlines are cascaded in series and duplicated in parallel. There are then ended with another two identical coupled-lines. The dual-path structure exhibits a 2^nd order dual-band response with three transmission zeros that separate the passbands from each other, and from the rejection band. The odd- and even-mode impedances of the coupled-lines control the bandwidth, passband and out-of-band responses of the dual-band filter. Finally, to verify the concept, a prototype of a dual-band bandpass filter is designed using EM simulator, fabricated on 1.6-mm-thick FR-4 substrate and measured. The resonant frequencies of the two passbands are centered at 1.82 GHz and 2.38 GHz.