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Altium DISCUSSION
When routing a differential interface in Altium, it is easy to focus on making the displayed lengths match. Which other checks should come before adding tuning patterns?
Consider a pair that changes layers and passes near a plane split. Could equal lengths still produce poor signal quality, and what information is needed before choosing a matching tolerance?
Equal physical length addresses only part of the timing problem. Check the interface's electrical requirements, the stackup, pair geometry, return path and layer transitions. A plane discontinuity or poorly controlled via transition can disturb the signal even when both tracks have identical lengths. The appropriate rules must come from the interface and component guidance, not from a universal matching number.
Distinguish intra-pair skew from matching between separate pairs or lanes. These requirements can be very different. Use the relevant timing budget and account for package and connector contributions when that information is available. If propagation is approximately 6 ps/mm on a particular structure, a 5 mm mismatch corresponds to about 30 ps; that estimate is meaningful only against a defined tolerance.
Excessive tuning can introduce closely coupled meanders and consume routing space without solving the dominant issue. Establish clean topology and return continuity first, then add only the tuning needed to meet the budget. Review the complete channel and use appropriate simulation or measurement when edge rates and channel complexity warrant it. DRC verifies geometric constraints, not the receiver's eye opening.
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