Differential Phase Skew Mitigation through Spread Glass Prepreg Selection
Spread glass prepreg selection eliminates differential phase skew by flattening fiber bundles and reducing dielectric constant variations across signal paths.
Inter-Pair Skew
Timing Offset
Differences in propagation delay between two independent differential pairs on a circuit board can lead to synchronization errors in parallel data interfaces. Managing inter-pair skew is essential for high-speed protocols like DDR memory and PCIe where multiple lanes must arrive within a specific time window. This variation is caused by differences in trace length and local dielectric properties.
It affects the overall timing budget of the system.
Geometric Factors
Routing paths that follow different routes across the board naturally experience different delays. Even when lengths are matched, inter-pair skew can arise from the fiber-weave effect if one pair traverses more glass bundles than another. Signal traces on different layers may also experience different phase velocities.
Careful planning of the layout is needed to ensure all pairs in a bus are electrically matched.
Performance Impact
Excessive timing differences between lanes can cause the data window to close, resulting in transmission failures. Systems with tight jitter requirements are particularly sensitive to inter-pair skew. Correction often involves adding serpentine traces to slower paths to match the fastest signal.
This technique must be executed without creating new impedance discontinuities. Simulation tools help predict these delays before the board is manufactured. Final validation is performed using high-bandwidth oscilloscopes on the assembled prototype.
The measurement of these offsets ensures the hardware meets the protocol specifications.