Quantifying the Impact of FEXT on Eye Margin for Coupled Lines in Inhomogeneous Media

Increase in printed circuit board costs is leading to denser routing of high-speed signal traces and to the use of low-cost dielectric materials for high-volume manufacturing. This, in turn, is increasing crosstalk among the traces. The crosstalk between the coupled traces in adjacent layers and the same layers is becoming an important factor to take into account as the signal speed increases. The dielectric media in the vertical stackup may not be homogeneous, and this increases the far-end crosstalk (FEXT) and insertion loss (THRU); thereby, reducing the signal to noise ratio of the high-speed link. This paper investigates the FEXT crosstalk impact on eye opening at a specified bit error rate at different signal speeds for differential coupled traces in inhomogeneous media and compares the results against homogeneous media models. A set of design guidelines regarding the material, coupled length, and stackup parameter selection is formulated for designers based on the signaling speeds.