Mode Symmetry Analysis and Design of CMOS Synthetic Coupled Transmission Lines

This paper presents the mode symmetry analysis and design of CMOS synthetic coupled lines that are meandered for the miniaturization design. The conventional even-odd mode analysis is not appropriate to apply due to the meandering structure, which is asymmetric. The asymmetric coupled-line model is therefore adopted for analyses and the equivalent model parameters are extracted based on ABCD-parameters converted from scattering parameters. The proposed extraction procedure is then applied on a 3-dB directional coupler as a design example. The results quantitatively show that symmetry of the coupled lines is well maintained. Symmetry of other coupled-line structures that degrade coupled-line symmetry, such as loosely coupled lines and different bend direction, are also analyzed based on this model. Finally, an equal-length bend design suitable for the CMOS synthetic coupled lines is proposed and has managed to improve symmetry on various structures.