PCB traces are lossy and long traces can degrade the signal quality. Traces must be kept short as possible. Unless mentioned otherwise, all signal traces should be 50Ω, single-ended impedance. Differential traces should be 50Ω, single-ended and 100Ω differential. Take care that the impedance is constant throughout. Impedance discontinuities cause reflections leading to EMI and signal integrity problems. Stubs must be avoided on all signal traces, especially the differential signal pairs. See Figure 8-10.

Within the differential pairs, the trace lengths must run parallel to each other and matched in length. Matched lengths minimize delay differences, avoiding an increase in common-mode noise and increased EMI.

Length matching is also important on MAC interface. All Transmit signal trace lengths must match to each other and all Receive signal trace lengths must match to each other. When using 1G transmission speeds, the tolerance for length matching is 50 mils. When using 100/10M, the tolerance for length matching is 100 mils.

Ideally, there should be no crossover or via on the signal paths. Vias present impedance discontinuities and should be minimized. Route an entire trace pair on a single layer if possible.

Figure 8-10 Avoiding Stubs in a Differential Signal Pair

Signals on different layers should not cross each other without at least one return path plane between them.

Coupling between traces is also an important factor. Unwanted coupling can cause cross talk problems. Differential pairs on the other hand, should have a constant coupling distance between them.

For convenience and efficient layout process, start by routing the critical signals first.