D-band transdirectional coupler employing an interdigital capacitor structure in 55-nm BiCMOS

Abstract

This work presents the first demonstration of a transdirectional (TRD) coupler operating at sub-THz frequencies. TRD couplers are particularly useful in active circuits due to their intrinsic DC blocking behavior, which is exclusive to this type of coupler. Despite the existence of techniques that enable high-frequency designs, no TRD coupler design has yet been demonstrated for frequencies above 30 GHz. This work therefore aims to demonstrate a TRD coupler operating within the D-band (110 GHz to 170 GHz), using an interdigital capacitor structure and considering a 55-nm BiCMOS technology. The proposed coupler's performance was verified through full-wave electromagnetic simulation and compared to other TRD designs from the literature. The proposed coupler occupies the smallest area and demonstrates comparable or superior performance across various parameters, with the exception of its quadrature behavior.