High-Speed PCB Design: The Basics

A circuit is high speed when its signals change fast enough that the board itself affects them.

What counts as high speed

It depends on how fast the signal edges rise, not only on the clock frequency. When the edges are fast compared with the trace length, the trace behaves like a transmission line.

Key practices

• Keep traces short and direct.
• Use a continuous ground plane under fast signals.
• Control trace impedance where the design needs it.
• Route differential pairs together and keep them the same length.
• Match the lengths of bus signals where timing matters.
• Avoid unnecessary vias and stubs.
• Place decoupling capacitors close to the chips.

Layers

High-speed boards often use four or more layers, so that solid ground and power planes sit close to the signals.

Plan early

High-speed requirements affect the layer stack and the layout from the start, so raise them at the beginning of the project.

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