Dual-band dual-mode bandpass filter using seriescoupled ring resonators

This paper proposed a dual-band ring filter using two identical microstrip dual-mode ring resonators. The rings are cascaded in series side-by-side via a single quarter wavelength coupled-line forming two transmission poles which are distributed in each passband. Meanwhile, the coupled-line positioned in between the two rings help to create the center transmission zero which can be used to control the rejection level of the in-between isolation. The characteristics of the dual-band filter in terms of bandwidth, passband and out-of-band responses are controlled by the line impedances of the rings and the odd-and even-mode impedances of the coupled-line. Finally, to verify the concept, a prototype of a dual-band dual-mode filter is designed using EM simulator, fabricated on 1.63-mm-thick Tachonic substrate and measured. The resonant frequencies of the two passbands are found at 1.73 GHz and 2.37 GHz while the center transmission zero is found at 2.04 GHz.