Temporal Matching
Geometric synchronization ensures that the two complementary signals in a high-speed link arrive at their destination at the same moment. Achieving differential pair phase alignment requires matching the physical length and electrical environment of both traces to within tight tolerances. This balance is necessary to prevent the conversion of differential signals into common-mode noise.
Skew Management
Differences in the arrival time of the two signals are known as skew, and they degrade the quality of the transmitted data. If one trace is longer than the other, the crossover point of the waveforms shifts, which reduces the timing margin at the receiver. Designers use serpentine routing to add small amounts of length to the shorter trace to correct this imbalance.
This length matching must account for the entire path, including vias and connector pins. Furthermore, the local environment of the traces must be identical, as a nearby mounting hole or a change in the glass weave can alter the signal speed on one line. Precision in the manufacturing of the substrate is just as important as the length of the copper.
Signal Integrity
Maintaining phase balance preserves the integrity of the differential eye diagram and maximizes the noise immunity of the link. When signals are perfectly aligned, the electromagnetic fields cancel each other out, which reduces radiated emissions. Any measurable mismatch increases the susceptibility of the circuit to external interference.
This precision is a requirement for standards such as PCI Express and Ethernet.
