Hybrid-coupler-based quasi-reflectionless balanced bandpass filter with all common mode suppression

Hybrid-coupler-based quasi-reflectionless balanced bandpass filter (BPF) is proposed and demonstrated in this paper. The reflectionless balanced BPF is comprised of two hybrid couplers and a filtering crossover. The filtering crossover is composed by a dual-mode ring resonator coupled with four short-circuited open-loop resonators, which is used to replace the traditional two BPFs for reduction of interference. Hybrid couplers are used for absorbing the reflected interference to the input and output ports. In order to achieve common-mode (CM) suppression, a double-layer structure with the same microstrip configuration on the upper and lower surfaces is adopted. The absorption characteristics of the differential mode (DM) and CM signals are optimized by hybrid couplers with improved ground. A third-order quasi-reflectionless balanced BPF with central frequency of 2.3 GHz and fractional bandwidth of 14 % is designed, fabricated, and measured. The final measured results demonstrate great consistency with the simulated ones.