High speed circuit design guidelines

A circuit is "high speed" when the signal rise time is short compared with the propagation delay of its traces (roughly when trace length exceeds one-sixth of the signal's rise-time length), so traces behave as transmission lines. The following points should be considered.

Guidelines

1. Transmission line effects and impedance control: keep trace characteristic impedance (Z₀) uniform (e.g. 50 ohm) using controlled width and dielectric height; avoid sudden changes (stubs, vias, connectors).
2. Termination: match the source and load to Z₀ using series, parallel or Thevenin terminations to avoid reflections, ringing and overshoot.
3. Ground and return path: use a continuous solid ground plane under signal traces; do not cross splits; provide close return vias.
4. Power distribution and decoupling: use power planes, place 0.1 uF (and smaller) capacitors close to each IC, and bulk capacitors; this reduces ground bounce and supply noise.
5. Crosstalk control: increase spacing (3W rule), avoid long parallel runs, put guard traces and route adjacent layers orthogonally.
6. Timing: equal-length routing for clocks and buses to limit skew; short clock traces; low-jitter clock.
7. Layout: short, direct traces with 45-degree bends instead of 90 degrees; minimise vias; keep high-speed parts together; separate analog and digital sections.
8. Component choice: pick parts with just enough speed (slower edges reduce noise and EMI), good packages (low lead inductance, surface mount).
9. EMI/EMC: shielding, filtering, ferrites and proper connector grounding.

Factors to consider

• Rise/fall time and bandwidth (f ≈ 0.35/t_r), dielectric constant of PCB (FR4 εr ≈ 4.5), propagation delay (roughly 170 ps per inch, about 6.7 ns per metre), number of layers and stack-up, power consumption and heat, cost, and testability.