Ring-resonator branch-line coupler with unequal power division

A ring-resonator branch-line coupler with unequal power division is proposed in this paper. The branch-line coupler with four coupled ring resonators at ports has the equivalent coupled-resonator filter network. The coupler therefore exhibits good stopband rejection. By carefully designing the coupling coefficients between resonators, the required unequal power division ratio along with good return loss and isolation could be obtained. A 2-GHz bandpass branch-line coupler of 5 dB power division ratio has been designed and fabricated on a Roger RO4003C substrate for verification. 20-dB return loss and isolation were achieved. Good consistency between simulated and measured results was also obtained.