Transmission Line Propagation Delay

Formula

Description

The propagation delay of a signal through a PCB trace depends on the trace length and the per-unit-length inductance and capacitance of the transmission line structure. For FR-4 microstrip, the delay is approximately 6-7 ns per meter (60-70 ps per cm). For stripline, it is about 7-8 ns per meter because the signal is fully immersed in dielectric. This delay is critical for timing analysis in high-speed digital systems, determining when signals arrive at their destinations and whether setup and hold times are met at receiving flip-flops.

Variables

• td — Propagation delay (s)
• length — Trace length (m)
• L_per_unit — Inductance per unit length (H/m)
• C_per_unit — Capacitance per unit length (F/m)

Practical Notes

For FR-4 microstrip: approximately 6.4 ns/m (64 ps/cm). For FR-4 stripline: approximately 7.2 ns/m (72 ps/cm). Propagation velocity = 1/√(LC) = c/√εr_eff. For microstrip, εr_eff ≈ (εr+1)/2 because half the field is in air. Length matching for differential pairs and clock distribution typically requires matching to within 5-50 ps depending on the data rate.