Correlating Glass Bundle Pitch Variation with High Frequency Phase Skew
Matching glass bundle pitch to trace geometry and using spread glass weaves eliminates local permittivity variations, bounding differential phase skew within tolerance.
Differential Skew Budget
Timing Tolerance
Propagation delay differences between the two conductors of a differential pair are governed by strict high-speed signal integrity guidelines. Known as the differential skew budget, this limit ensures that the complementary signals arrive at the receiver nearly simultaneously. Excessive delay discrepancy between the true and complement lines degrades the eye diagram and increases common-mode electromagnetic radiation.
The budget is typically defined in picoseconds or as a small percentage of the signal unit interval.
Imbalance Cause
Physical variations in the printed circuit board substrate and layout routing represent the primary sources of timing asymmetry. When designing high-speed boards, a differential skew budget must account for trace length mismatches and the fiber-weave effect of the fiberglass cloth. As signals travel through the glass bundles and epoxy resin regions of the laminate, they experience different dielectric constants, which changes the signal speed along each line of the pair.
This variation becomes critical in multi-gigabit per second channels, where even a few mils of length difference can consume the entire available budget. Consequently, trace layouts must be carefully matched in both length and environment.
Mitigation Protocol
Designers employ precise layout techniques to maintain differential signal alignment within the allocated limits. Adjusting the lengths of the traces through the addition of small serpentine segments helps maintain the differential skew budget. Specifying spread-glass laminates also reduces dielectric variations.