Miniaturized quadruple-mode periodic stepped-impedance coupled-ring resonator bandpass filter with a sharp transition band and a wide stopband

In this paper, we aim at developing a quadruple-mode bandpass filter based on periodic stepped-impedance coupled-ring resonator. Based on the dual-mode periodic stepped-impedance ring resonator design in [J.-T. Kuo and C.-Y Tsai, 2006], a coupled-ring resonator with perturbation on the symmetric plane is devised. The resonant characteristics of the double-ring resonator structure are investigated in [I. Wolff and W. Menzel, 1975]. In [U.-H. Lok et al., 2008], the coupled-ring structures are employed to contrive novel quadruple-mode bandpass filters. The circuit achieves low insertion loss, relatively wide passband and sharp transition band. The proposed resonator structure developed here exhibits four degenerate resonances and four transmission zeros. Analysis of the resonant frequencies is carried out by taking advantages of structure symmetry.