Analytical Prediction
Computational simulations predict the arrival time differences between signals in a high speed differential pair. Designers use phase skew modeling to account for the physical and electrical variations that prevent the two halves of a pair from remaining perfectly synchronized. This analysis is necessary for maintaining the common-mode rejection that protects the signal from external noise.
Fiber Influence
Woven glass reinforcements create a periodic variation in the local dielectric constant of a substrate. In phase skew modeling, the software must account for how a trace interacts with the glass bundles and the resin-rich windows between them. If one trace of a pair sits on a glass bundle while the other sits on resin, they will travel at different speeds.
This glass weave effect is a major source of timing jitter in systems operating at data rates above 5 Gbps. Advanced models use statistical distributions of the weave pattern to estimate the worst-case skew that might occur in a production lot.
Trace Compensation
Corrective design techniques minimize the impact of these timing mismatches. Once phase skew modeling identifies a potential problem, engineers can implement zig-zag routing or use spread-weave glass fabrics to average out the dielectric variations. Some designers also specify a slight rotation of the entire circuit layout relative to the glass weave to ensure that no trace stays aligned with a single glass yarn for a long distance.
Validating these models requires measuring the actual time-of-flight differences on fabricated test coupons using a high bandwidth oscilloscope.
