Quadruple-mode coupled-ring resonator bandpass Q filter with quasi-elliptic function passband

Quadruple-mode coupled-ring resonator is used to design quasi-elliptic function bandpass filters (BPFs) with a relatively wide bandwidth. The resonator consists of a pair of coupled rings with two open stubs symmetrically tapped to the inner ring. The stubs are used to move a transmission zero near the center frequency to lower transition band. It is believed that it is the first ring resonator BPF based on the coupled-ring to create four transmission poles and four transmission zeros. Two filters with relative bandwidths of 10% and 15% are fabricated and measured. Good agreement between simulation and measurement is obtained.