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Causes of Signal Degradation in PCB Transmission Lines


Atar Mittal
Sierra Circuits

Skill Level

 0 min 0 sec video

Signal degradation on PCB transmission lines manifests in many forms: undershoot, overshoot, ringing, pulse shape distortion, switching noise, attenuation, ground bounce, skew, etc. All these can be attributed to one or more of these sources: signal reflections caused by characteristic impedance discontinuities; signal distortion due to conductor and dielectric losses resulting from PCB materials’ properties as signals travel over the transmission lines; crosstalk from signals on nearby PCB conductors; noise in power distribution network; electromagnetic interference (EMI). Impedance discontinuities manifest from many sources, to name a few: unmatched loads and terminations, non-uniformity in the lines, vias, stubs, component and test pads, gaps in reference planes and poorly designed return paths, stray capacitances and inductances, and branching of signal paths, and all these cause signal reflections. Frequency dependence of the copper and dielectric losses cause unequal attenuation of various frequency contents in the signals, causing signal rise time degradation, and variations in dielectric constant with frequency cause different frequency signal components traveling at different speeds.

Crosstalk from nearby conductors occurs due to inductive and capacitive coupling, causing several issues: near- and far-end crosstalk, switching noise, ground bounce, etc. PDN noise and unwanted electromagnetic energy will superimpose on signals causing signal integrity issues. After identifying the root cause, one can find solutions to the signal integrity problem.

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