A Coupled-Line Marchand Balun with Symmetric Inductive Compensation for High-Impedance Transformation

This paper presents a single-layer coupled-line Marchand balun designed for a nominal transformation from a 50-Ω unbalanced source to a 300-Ω differential load at 2.25 GHz. The circuit combines two inductively terminated coupled-line sections with an independent input matching network. Symmetric termination inductors provide a means of tuning the coupled-line response without introducing additional cascaded sections. The design was developed using even- and odd-mode transmission-line analysis and optimized in Keysight Advanced Design System (ADS). Conventional and compensated prototypes were fabricated for comparison. At 2.25 GHz, the compensated prototype yielded a reported input reflection level of −28.05 dB, with output transmission levels of −3.11 and −3.05 dB. Its reported amplitude imbalance was 0.052 dB, and the output phase difference was 180.52∘ (0.52∘ phase error). The corresponding conventional prototype exhibited an input reflection level of −21.51 dB, an amplitude imbalance of 0.217 dB, and a phase error of 9.29∘. The results indicate improved balance and input matching at the design frequency. Because the retained records do not establish whether the VNA data were renormalized from physical 50-Ω ports to the intended 150-Ω output references, the measurements cannot independently confirm the differential impedance transformation. Full-band performance, common-mode rejection, and circuit-area advantages are therefore not quantified.