Printed Circuit Design & Fab Online Magazine - Current Issue

Designer’s Notebook

Lance Wang How to calculate trace length from time delay value for high-speed signals.

To keep a good high-speed signal quality from driver to receiver on a PCB is not an easy task for designers. One of the most challenging issues is managing the propagation delay and relative time delay mismatches. To manage the time delays, we need to know how to calculate trace length from time delay value in order to implement the PCB trace routing accordingly. Let me take you through the process.

Calculating signal speed. According to physics, electromagnetic signals travel in a vacuum or through the air at the same speed as light, which is:

Vc = 3 x 10⁸M/sec = 186,000 miles/sec = 11.8 in/ns

Read more: Managing Time Delays

What to do when the trace doesn’t act as an “ideal” connection.

Read more: The Physics of Transmission Lines

A mismatch in trace symmetry can cause noise coupling or timing issues.

Read more: Signal Integrity Design Fundamentals

Ralf Bruening Exact adaptations of impedance are often not necessary. Instead, minimize impedance deviations.

In part 1 last month, we took a back-to-basics approach and discussed line impedance and its effects on signal integrity. Every electrical conductor comprises capacitance, an inductance, and a frequency-dependent ohmic resistance. With increasing frequencies, these electrical characteristics will influence and distort the signal.

Applying a transmission line model based on the telegrapher’s equations (as typically common in signal integrity considerations, except for when considering extremely high data rates, e.g., Serdes channels), one often-used general expression for the characteristic impedance of a lossy transmission line is:
designersequation1 Eq 1