EMI Bites: Signal Currents Don’t Wait at the Load
Many engineers still imagine current flowing to the load and only returning after the signal arrives.
I used to be one of them, but that’s not how real signal propagation works.
In reality, signals travel as electromagnetic fields through the dielectric, and the return current forms instantly, following the signal in real time.
Why this matters:
- As the signal transitions (e.g., from 0 V to 3.3 V), it creates a displacement current in the surrounding dielectric.
- This displacement current closes the loop continuously along the trace—not just at the end.
- The return current follows the signal wavefront, creating a dynamic loop that exists at every point along the route.
- Misunderstanding this leads to broken return paths, field spreading, and EMI problems.
Key Insight:
The return path moves with the signal—not behind it.
How to design better return paths:
- Place reference planes (RRPs) directly adjacent to signal layers to keep loops tight.
- Avoid splits or gaps in reference planes—use also return and reference vias (RRV) to maintain continuity.
- Think in terms of field propagation, not just trace currents.
- Use field solvers or EMI simulation tools to visualize return current paths and optimize layout decisions when needed.
Hope this helps,
Dario
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