Master the art of routing USB, PCIe, Ethernet, and SerDes interfaces. From material selection to channel simulation, learn to design PCBs that meet modern signal integrity requirements.
| Interface | Data Rate | Zdiff | Loss Sensitivity | Recommended Material |
|---|---|---|---|---|
| USB 2.0 | 480 Mbps | 90Ω | Low | FR-4 |
| USB 3.0/3.1 | 5/10 Gbps | 90Ω | Medium | FR-4 / Mid-Loss |
| USB 3.2/4 | 20/40 Gbps | 85Ω | High | Low-Loss |
| PCIe Gen3 | 8 GT/s | 85Ω | Medium | FR-4 / Mid-Loss |
| PCIe Gen4 | 16 GT/s | 85Ω | High | Mid-Loss / Megtron |
| PCIe Gen5 | 32 GT/s | 85Ω | Critical | Megtron 6/7 |
| 1GbE | 1 Gbps | 100Ω | Low | FR-4 |
| 10GbE | 10 Gbps | 100Ω | Medium | FR-4 / Mid-Loss |
| 25GbE | 25 Gbps | 100Ω | High | Megtron 6 |
| 100GbE (4x25G) | 100 Gbps | 100Ω | High | Megtron 6/7 |
A trace acts as a transmission line when its length exceeds 1/10 of the signal's wavelength (λ/10) or when the propagation delay is significant compared to the signal's rise time. Rule of thumb: for rise times < 1ns, treat traces > 1 inch as transmission lines. For modern high-speed signals, almost all traces are transmission lines.
NRZ (Non-Return-to-Zero) uses 2 levels and is simpler but doubles the Nyquist frequency with data rate. PAM4 uses 4 levels, halving the Nyquist frequency but requiring better SNR. Above 28 Gbps, PAM4 is typically used because channel losses at NRZ Nyquist become prohibitive. PAM4 requires 9.5 dB better SNR than NRZ.
The 3W rule states that the edge-to-edge spacing between traces should be at least 3× the trace width to reduce crosstalk to acceptable levels (~10%). For aggressive crosstalk targets (<5%), use 4W or more. This rule applies to single-ended traces; differential pairs have their own coupling requirements.