D-Band Four-Way Differential Power Divider/Combiner Using Transformer Coupling

A D-band four-way differential power divider/combiner is proposed using a transformer-coupled tri-port architecture with an embedded capacitive isolation network. Unlike conventional cascaded two-way approaches, the proposed topology establishes differential-mode balance and impedance normalization at the first tri-port stage, which suppresses the accumulation of phase and amplitude errors in subsequent division stages. Specifically, a capacitive isolation network connecting the in-phase nodes is introduced to selectively attenuate undesired even-mode coupling, resulting in significantly improved differential isolation and phase accuracy without employing lossy elements. A design methodology, including parameter selection guidelines for the tri-port transformer and isolation capacitance, is also presented, along with a mode-based analysis. Implemented in 65-nm bulk CMOS, the proposed divider/combiner occupies a compact core area of$0.26\times 0.14$mm2. It demonstrates a minimum insertion loss of 1.89 dB with differential isolation exceeding 17 dB over the entire D-band.