On Mode Conversion in Geometrically Unbalanced Differential Lines and Its Analogy With Crosstalk

In this paper, mode conversion occurring in differential lines with cross section affected by geometric imbalance is investigated by recognizing a strict analogy with crosstalk, and by rephrasing in the modal domain the assumption of weak coupling (usually exploited for crosstalk analysis) in terms of weak imbalance. The derivation focuses on the conversion of the differential mode signal into common mode (CM) noise responsible for undesired radiated emissions, and leads to an approximate circuit model for CM-current prediction, in which the effects due to mode conversion are included by distributed voltage and current sources. Model accuracy is assessed by experimental and numerical validations involving unbalanced microstrip lines.