11.1.1 Signal Traces

PCB traces are lossy and long traces can degrade signal quality. Traces should be kept short as possible. Unless mentioned otherwise, all signal traces should be 50-Ω, single-ended impedance. Differential traces should be 50-Ω single-ended and 100-Ω differential. Take care to ensure impedance is controlled throughout. Impedance discontinuities will cause reflections leading to emissions and signal integrity issues. Stubs should be avoided on all signal traces, especially differential signal pairs.

Figure 142. Differential Signal Trace Routing

Within the differential pairs, trace lengths should be run parallel to each other and matched in length. Matched lengths minimize delay differences, avoiding an increase in common mode noise and emissions. Length matching is also important for MAC interface connections. All transmit signal traces should be length matched to each other and all receive signal traces should be length matched to each other.

Ideally, there should be no crossover or vias on signal path traces. Vias present impedance discontinuities and should be minimized when possible. Route trace pairs on the same layer. Signals on different layers should not cross each other without at least one return path plane between them. Differential pairs should always have a constant coupling distance between them. For convenience and efficiency, TI recommends routing critical signals first (that is, MDI differential pairs, reference clock, and MAC IF traces).