Differential-Line Characterization Using Mixed-Port Scattering Parameters

High-speed transmission lines are commonly routed as differential lines to control sensitivity to noise on the reference planes at higher speeds. Differential lines are typically characterized in terms of mixed-mode scattering parameters, as they provide insight into the behavior of differential and common signals, as well as the mode conversion among them. These mixed-mode scattering parameters can be mathematically obtained from single-ended parameters, which can for example be measured with a 4-port vector network analyzer. There has been recent efforts to develop extended or modified versions of mixed-mode scattering parameters, especially for tightly-coupled lines. This can be a point of confusion in interpreting the behavior of differential lines. In this paper we introduce the mixed-port scattering parameters, which do not suffer from any such ambiguous definitions. Mixed-port scattering parameters are defined based on common and differential port excitations, allowing a simpler interpretation than their mixed-mode counterparts, without the need for defining even, odd, common, or differential-mode impedances. As such, mixed-port scattering parameters can be used to analyze the performance of a general differential network, certainly including coupled or asymmetrical lines, without any ambiguity.