An Ultra-Compact and Wideband Transformer-Based Coupler With Arbitrary Phase Difference and Arbitrary Power Division Ratio

This article presents a wideband ultra-compact transformer-based lumped coupler that offers an arbitrary phase difference and power division ratio (PDR) between the two output ports, while maintaining input-output matching and isolation conditions. The transformer-based lumped design of the coupler ensures a compact layout and is conducive to ON-chip implementations. In addition, the inherent high-order network nature of the physical transformer achieves wideband performance. The design theory of the proposed coupler is analyzed and the closed-form design equations for the values of the components are derived. As a proof of concept, four different 45° / 60° prototypes with equal and unequal PDRs are implemented in GlobalFoundries 22-nm CMOS FD-SOI technology at 55–60 GHz. The measurements of the coupler prototypes are in good agreement with the electromagnetic (EM) simulation results and the performance predicted by the theoretical analysis. They demonstrate that the proposed transformer-based coupler with arbitrary phase difference and arbitrary PDR is capable of achieving at least
$14\times $
smaller size compared to previously reported transmission-line/coupled-line implementations and around 15%–20% larger fractional bandwidth (FBW) compared to previously reported lumped coupler implementations.