A transformer-based poly-phase network for ultra-broadband quadrature signal generation

This paper presents a novel transformer-based poly-phase passive network for generating high-quality fully differential quadrature signals with low-loss, ultra-broad bandwidth, and high compactness. A differential folded transformer quadrature coupler serves as the building block in the proposed poly-phase network. The poly-phase network can be readily extended to high-order configurations for broadband operation. Unlike the RC-CR poly-phase network, the transformer-based design offers inherently low loss and compatibility with output loading. As a proof-of-concept design, a 3-stage transformer poly-phase network is implemented in a standard 65 nm bulk CMOS process and achieves high-quality differential quadrature generation over a decade bandwidth. Measurements demonstrate the maximum differential quadrature outputs phase error within ±2° from 2 GHz to 20 GHz, maximum magnitude mismatch within ±1 dB from 2.6 GHz to 25 GHz, and a worst-case image rejection ratio of 25 dB. The measured passive loss is 3.5 dB at 6.4 GHz.