Signal integrity

Signal integrity is about what happens when a trace stops acting like a simple wire. Every trace takes time for a signal to cross. When an edge is fast enough that it’s still rising while the wavefront travels the trace, the trace behaves as a transmission line, a structure with its own characteristic impedance, and wave effects show up: reflections, ringing, overshoot.

The counterintuitive rule is that edge rate matters, not clock rate. A humble 8 MHz microcontroller whose output drivers switch in a nanosecond produces edges with frequency content into the hundreds of megahertz, fast enough to ring on a long trace. You don’t need a high-speed design to have high-speed problems. Fast edges and enough distance will do it.

The other classic failure is crosstalk. A switching trace couples capacitively and inductively into its neighbors and leaves a copy of its signal on theirs. It gets worse with long parallel runs and tight spacing. The defenses are geometric: more space between aggressor and victim, shorter parallel runs, and a solid ground plane close underneath, which shrinks the coupling fields dramatically.