Compact 2–18 GHz on‐chip balun based on centrosymmetric spiral transformers

Abstract

A novel broadband balun based on centrosymmetric spiral transformers is proposed. Two parallel circular spiral transformers comprise the proposed balun, which is different from the Marchand structure. The equivalent coupled line model demonstrates that asymmetric structure is helpful to realise the appropriate balun with good amplitude balance and low return loss. Finally, combined with the structural characteristics of the spiral transformer in monolithic microwave integrated circuit designs, a centrosymmetric structure with compact size is adopted. The balun is designed and fabricated in WIN's IP2M process and its core size occupies an area of 700 μ m × 300 μ m . The simulation and measurement agree well, exhibiting a bandwidth covering from 2 to 18 GHz with good return loss, flat insertion loss and high stability of 180° phase difference.