A Novel Compact Uniplanar Isolation Circuit for Three-Port Baluns

This paper presents a novel compact planar isolation circuit (IC) based on transformer for outstanding isolation and matching at all ports of a three-port balun. In this IC, a compact transformer is employed instead of a bulky
$\lambda/2$
transmission line to generate out-of-phase currents between the two balanced ports. This transformer requires only a low coupling factor of 0.5, and is thus easily implemented on a single metal layer. The closed-form equations for the design of the IC are proposed and analyzed. To validate the authors' design concept, a prototype patterned-ground planar Marchand balun with the proposed IC at center frequency of 2.5 GHz is designed, fabricated and measured. The balun has an area of 194 mm
²
, where the proposed IC is as small as
$0.039 \lambda_0 \times 0.047 \lambda_0$
. The balun features excellent amplitude and phase balance, along with high isolation of up to 37.7 dB between the two balanced ports and high return losses above 28.8 dB at all ports. Good agreement between the simulated and measured results proves the correctness of the theory.