Transmission line issues in high speed design

A trace behaves as a transmission line when the signal rise time is comparable to or shorter than twice the propagation delay of the trace (roughly length > t_r/(2·t_pd)). Then voltage and current exist as travelling waves, and the following issues must be considered.

1. Characteristic impedance (Z₀) and impedance matching: Z₀ = √(L/C) depends on trace width, dielectric thickness and εr. Source, line and load should be matched to avoid reflections.
2. Reflections: a mismatch (Γ = (Z_L - Z₀)/(Z_L + Z₀)) reflects part of the wave, producing ringing, overshoot and undershoot that can create false triggering. Use series or parallel termination.
3. Propagation delay: t_pd = √(μ·ε) per unit length (about 170 ps per inch on FR4); it affects timing, so delay-critical lines must be short.
4. Skew: unequal delay between lines of a bus or differential pair; match lengths.
5. Crosstalk: coupling between adjacent lines; control the spacing and use ground planes.
6. Impedance discontinuities: stubs, vias, bends, connectors, plane splits and width changes.
7. Attenuation and dispersion: conductor (skin effect) and dielectric loss at high frequency, which slows edges.
8. Ground bounce/return path and EMI: a continuous return path under the trace is required.
9. Termination methods: series (source) termination, parallel, Thevenin and AC termination; the choice affects power and speed.