Signal Integrity — Transmission Lines, Crosstalk & Via Modeling
Characteristic impedance, S-parameter extraction for vias and connectors, differential pairs, eye diagram analysis, and high-speed PCB design rules.
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Quick Explainer
What is characteristic impedance and why does impedance mismatch cause reflections?
Characteristic impedance Z0 is determined by PCB trace geometry and dielectric constant. When a signal hits a discontinuity (via, connector, width change), the impedance changes. Reflection coefficient Gamma = (ZL - Z0)/(ZL + Z0). In a 50-ohm system, a via presenting 80 ohms reflects 23% of the signal — enough to cause glitches at multi-GHz data rates.
How does FEM extract S-parameters of a via for signal integrity analysis?
Create a 3D model of the via with excitation ports at each signal entry/exit. A full-wave FEM solver computes fields over a frequency sweep and extracts S-parameters. These are imported into SPICE or ADS as an N-port model, cascaded with driver/receiver and transmission line models to simulate the complete signal path and generate eye diagrams.