A 60GHz Cross-Coupled Transformer-Based Quadrature-Phase Coupler in 22nm FD-SOI

This paper presents and analyses a transformer-based quadrature-phase coupler using a capacitive cross-coupling technique. The transformer-based architecture is preferable for circuit designs for quadrature-phase generation at high frequencies due to its compact size and low loss. However, this approach could suffer from the low magnetic coupling between metal conductors, which is limited by the minimal distance of metal conductors in the technology available. A low magnetic coupling will cause an insufficient coupling coefficient of the coupler leaving an amplitude imbalance at the coupler outputs, which is bad if an equal power division is desired. A novel capacitive cross-coupling technique is introduced in this work. By adding two cross-coupled capacitors diagonally, the overall coupling coefficient of the coupler can be improved. To prove the concept, a 60 GHz circuit is implemented in 22nm FD-SOI (fully-depleted silicon on insulator) CMOS. The measured insertion loss of the proposed coupler is 1.3dB. The measured phase difference between the two output ports is 92° at 60GHz. The coupler achieves an isolation between two outputs of around 30dB at 60GHz. To the best knowledge of the authors, this is the first cross-coupled transformer-based quadrature-phase coupler operating at 60GHz.