Optimal Design of Broadband Microwave Baluns Using Single-Layer Planar Circuit Technology

A new design methodology is presented for the planar implementation of a classical Marchand balun. A novel intuitive analysis shows that the Marchand configuration can be designed optimally to eliminate the phenomenon of "trace separation," which is frequently observed in planar implementations. The new theory shows that this unbalancing effect is caused by the parasitic transmission line formed between the inner strip and ground, which is not considered in Marchand's original coaxial structures. Compact design equations are derived, based on which a new innovative structure is proposed and fabricated. This demonstrates the elimination of trace separation and achieves flat equal port split over a double octave bandwidth, performing up to 10 GHz, using an industry standard single-layer thin-film process having a continuous unpatterned ground plane. Popular planar variants of Marchand's original structures are also designed and fabricated to verify the new design equations. These structures are compared in terms of bandwidth, trace separation, and balanced port impedances.