Issue link: https://iconnect007.uberflip.com/i/1369942
MAY 2021 I DESIGN007 MAGAZINE 31 3. Gaps in The Return Plane Breaks or gaps in the return path are major causes of radiated emissions, radiated suscep- tibility, and ESD failures. Let's come back to the issue of a gap or slot in the return plane mentioned earlier and show an example of why it's bad news for EMI. When the return path is interrupted, the conduction current is forced around the slot, or otherwise finds the nearest (lowest impedance) path back to the source. e electromagnetic field is forced out and the field will "leak" all over the board. I have an article and good demonstration video of this and how it affects common mode cur- rents and, ultimately, EMI (Figure 11). [7] is would be a great demo to construct and show your own colleagues! e difference between the gapped and un- gapped traces is shown in Figure 12. Note the harmonic currents are 10-15 dB higher for the gapped trace (in red). Failing to pay attention to the signal and power return paths is a major cause of radiated emissions failures. 4. Power Distribution Network Design Power distribution network (PDN) design requires a low impedance (0.1 to 1.0 ohms, typically) transmission line through at least 30 MHz. e purpose of a PDN is to transfer energy from the power source (oen a volt- Figure 11: Shows a demonstration test board with transmission lines terminated in 50 ohms. One transmission line has a gap in the return plane and the other doesn't. A 2 ns pulse generator is con- nected to one of the two BNC connectors in turn and the harmonic currents in a wire clipped to the return plane are measured with a current probe. Figure 12: The resulting common mode currents on an attached wire as measured with a current probe. The trace in aqua is the un-gapped return path and the trace in red, the gapped return path. The difference is 10-15 dB higher for the gapped return path. These harmonic currents will tend to radiate and will likely cause radiated emissions failures.