Transformer design for 77-GHz down-converter in 28-nm FD-SOI CMOS technology

This paper presents a comparative analysis of integrated transformers for a 77–GHz down-converter in a 28-nm fully depleted (FD) silicon-on-insulator (SOI) CMOS technology. The proposed down-converter, which is addressed to long-range automotive radar applications, is based on a fully differential mixer-first architecture and exploits two integrated transformers, i.e. an input transformer for single-ended-to-differential conversion of the 77-GHz signal and an inter-stage transformer to feed a current-driven passive Gilbert-cell. Both transformers have been properly designed, while exploiting the most suitable spiral configuration to meet the stringent requirements of automotive applications. To this aim, stacked, interleaved, and interstacked transformers have been compared by means of extensive electromagnetic simulations at 77-GHz. The comparison has been carried out in terms of insertion loss (IL) and transformer characteristic resistance (TCR), which are the most suitable figures of merit. The interstacked configuration provides the lowest IL (i.e., 1.2 dB at 77 GHz), thus resulting the best choice as input balun. The interleaved topology has been chosen instead as inter-stage transformer thanks to its high TCR (i.e., 1.9 kΩ at 77 GHz), which leads to better conversion gain.