Dual-band bandpass ring filter

A topology of a dual-band filter is presented based on a series-combination of two identical ring resonators. The circumference of each ring is equal to one wavelength taken at the center frequency between the two chosen operating frequencies. The control of the filter response in terms of bandwidth and separation between the operating frequencies can be done by varying the impedances of the filter elements which are the ring line impedance and the even- and odd-mode impedances of the coupled lines. The proposed concept is demonstrated through a design of a microstrip dual-band filter operating at 0.9 GHz and 1.8 GHz on epoxy substrate with ε_r = 5.4 and substrate thickness of 1.6-mm. Experimental results show good agreement with the theoretical analysis.