Transmission-Line Critical Length

Formula

Description

A PCB trace must be treated as a transmission line (with controlled impedance and termination) once its length exceeds the critical length, where the round-trip propagation delay becomes comparable to the signal rise time. Below this length the trace behaves as a lumped node and reflections settle within the edge; above it, reflections cause ringing and must be managed.

Variables

• lcrit — Critical (electrically-long) length (m)
• tr — Signal rise time (s)
• tpd — Propagation delay per unit length (s/m)

Practical Notes

A common rule uses tr/(2·tpd); some designers use tr/(6·tpd) for a stricter limit. FR-4 microstrip has tpd ≈ 6–7 ps/mm, so a 1 ns edge gives a critical length around 75 mm.