A CMOS 3-dB directional coupler using edge-coupled meandered synthetic transmission lines

Design of CMOS 3-dB directional coupler by incorporating the edge-coupled meandered synthetic quasi-TEM transmission lines (TLs) is presented. The proposed synthetic TL, so-called the complementary-conducting strip coupled-line (CCS CL) consists of the unit cell, which has more design parameters than that of the conventional thin-film coupled-line (TF CL). The extensive theoretical analyses shows the proposed meandered CCS CL can break the process limitations on straight TC CL, achieving the coupling coefficient (k-value) with 10.8% improvement. The prototype of the 3-dB directional coupler is designed based on the proposed CCS CL and fabricated in an area of 120.0 × 240.0 μm (without contact pads) based on the standard 0.18-μm 1P6M CMOS technology. The experiments show the prototype can cover a bandwidth from 14.2 GHz to 36.9 GHz with a coupling of 4.5 dB. The literature survey shows that the presented design achieves the tight coupling with the highest efficiency on the circuit miniaturization.