Differential Phase Skew Verification Using High Bandwidth Time Domain Reflectometry
High bandwidth TDR isolates sub picosecond differential phase skew on test coupons to prevent glass weave induced mode conversion in high speed channels.
Stripline Skew
Timing Offset
Propagation delay variation between conductors in a differential pair arises from dielectric non-homogeneity or length mismatch. High speed digital signals experience stripline skew when one half of a differential signal arrives at the receiver later than the other. This timing error degrades the eye diagram and increases bit error rates.
Glass Geometry
Microscopic variations in the weave of the fiberglass reinforcement cause local changes in the dielectric constant. A trace running over a glass bundle travels faster than a trace running over a resin-rich area. Using spread glass weaves or rotating the circuit layout by a few degrees helps to average out these local differences.
Signal Compensation
Designers mitigate these timing offsets by adding small serpentine traces to the faster path to equalize the electrical length. Precision routing software calculates the required delay based on the material properties and the signal frequency. At speeds exceeding 10 Gbps, even a few picoseconds of difference can cause common-mode noise and electromagnetic interference.
Tight control over the manufacturing tolerances of the dielectric thickness and trace width is necessary for maintaining signal integrity. Testing with a time domain reflectometer confirms that the skew remains within the functional limits of the receiver.