Propagation Delay

Propagation Delay

Signals don't travel instantaneously through PCB traces. The propagation delay depends on the dielectric constant (Dk or εr) of the PCB material.

t_pd = (√εr) / c

Where:

• t_pd = delay per unit length (seconds/meter)
• εr = relative permittivity (dielectric constant)
• c = speed of light = 2.998 × 10⁸ m/s

Common PCB materials:

For a microstrip (trace on outer layer), the effective εr is lower than the bulk value because part of the field travels through air. A common approximation:

εr_eff ≈ (εr + 1) / 2

Total delay for a trace of length L (meters):

T = t_pd × L = L × √εr / c

Your Task

Implement propagationDelay(lengthMm, er) that returns total delay in picoseconds (ps) using the effective dielectric constant approximation for microstrip.