Mode Conversion Due To Residual Via Stubs in Differential Signaling

Mode conversion is an important source of degradation in differential links, which can be induced by a trace length mismatch or asymmetric ground via configurations. This paper explores another source of mode conversion caused by asymmetric residual via stubs. These remnant via stubs can be present because of multiple reasons, such as residual stubs due to process tolerances or errors during back-drilling. Different via configurations have been analyzed through full-wave simulations. Results show a large impact on mode conversion with an increment of around -35 and -15 dB when the residual stub difference changes from 2 to 14 mil at the fundamental frequency of 17.5 GHz. Furthermore, an estimation approach of mode conversion as a function of via stub asymmetry is proposed.