Comparative Study on De-embedding of Highly Assymetrical Differential Devices using Multimode TRL and Applicability of Mode Separation

Differential signalling offers numerous advantages at high frequencies. Especially in passive symmetric devices, where differential and common modes propagate with minimal coupling, independent mode analysis using cascade-based twoport de-embedding techniques is common practice. This principle has also been adapted for de-embedding of asymmetrical structures under assumption of small or moderate mode coupling, as it is the case in most error-boxes in on-chip de-embedding tasks. This significantly s implifies de -embedding an do es not require the use of advanced four-port multimode-TRL (MTRL). In this paper we systematically analyze the limits of the mode separation and two-port TRL technique by applying it to various error-box structures on PCB with different asymmetries, such as variations in line width and length. We show empirically beyond which asymmetry level, characterized by the mode conversion, the usage of the full four-port techniques is mandatory and the mode separation technique combined with simple two-port TRL is no longer valid.