Microstrip balanced filter using interlocked half/quarter-wave resonators with improved CMRR by reducing the common-mode coupling

A microstrip balanced filter using interlocked half-wave (λ/2) and quarter-wave (λ/4) resonators with improved CMRR has been proposed in this paper. By attaching properly chosen capacitors along the bisected symmetric line of the filter structure, the common-mode transmission can be significantly reduced thus achieving satisfactory CMRR among passband. The selection guideline is described according to the current density distribution of the balanced filter under common-mode excitation. Specifically, a CQ balanced filter with quasi-elliptic response is designed and implemented, verifying the validity of the proposed approach.