Ultra-wideband uniplanar MMIC balun using field transformations

A new wideband three-port balun is developed using a standard uniplanar monolithic microwave integrated circuit (MMIC) process. This structure does not rely on bulky resonant circuits, and thus can be realised in a small size and show ultra-wideband characteristics. Out of phase characteristic between two output ports is inherently achieved by field transformations through physical transitions of uniplanar transmission lines. A fabricated back-to-back balun pair showed insertion loss less than 1.85 dB and return loss more than 10 dB over an ultra-wideband frequency range from 1 to 92.5 GHz.