Miniaturized microstrip quadruple-mode ring resonator filter with quasi-elliptic passband

A compact quadruple-mode ring resonator is proposed for synthesis of a quasi-elliptic bandpass filter. The resonator is composed of four half-wavelength rings connected to a single grounded via. For miniaturization, the ring areas are overlapped and the crossovers are implemented by short CPW sections in the ground plane. As a result, the resonator uses about 50% area of a conventional dual-mode ring resonator filter. Among the four rings, all coupling coefficients are of inductive type realized by shorted high-impedance and coupledline sections. Two zeros are created by capacitive source-load coupling, and the response is synthesized by the coupling matrix method. Agreement between simulation and measurement results of a realized circuit validates the approach.