Dual-band dual-ring bandpass filter with large bandwidth ratio

A novel dual-band bandpass filter with large bandwidth ratio is proposed by using two dual-mode square ring resonators. By stretching the length of the loaded open stub of the inner ring, one of its first-order degenerate modes moves down and forms the first passband together with a pair of first-order degenerate modes of the outer ring resonator. Meanwhile, the second passband is constructed by another first-order degenerate mode of the inner ring resonator and its second-order degenerate mode. In this way, the realisable fractional bandwidth of the first passband can be largely enhanced with the new combination of multiple resonant modes. Finally, a prototype with a high fractional bandwidth ratio of two passbands has been realised and verified experimentally.