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FEBRUARY 2026 I I-CONNECT007 MAGAZINE 109 microstrip traces don't inherently radiate; only the ones with poorly designed return paths do. The most prevalent form of common-mode radia- tion emanates from system cables, which readily behave like monopole antennas. To limit the resulting common-mode current, it's important to: • Limit the driver: Reduce the driver current strength. • Tighten the return path: Keep the signal and return (reference plane) in very close proximity. Avoid plane splits, slots, or layer transitions that force long, wandering return paths. • Improve PDN and decoupling: Design a low- impedance PDN (tight power-ground spac- ing, proper decoupling). Use low-inductance mounting and reduce the loop area of the decoupling capacitors. • Adopt an effective grounding strategy: Use solid, continuous ground planes. Employ a single, low-impedance connection between I/O ground and chassis/enclosure ground. Avoid ground islands and poorly thought-out star-routed grounds in high-speed sections. • Tame edge rates and spectra: Use slew-rate control where possible to slow unnecessarily fast edges. Consider spread-spectrum clocking to reduce peak energy at discrete frequencies. • Optimize differential pairs: Maintain good balance with matched impedance, aligned delays, and symmetric routing. Avoid unnec- essary asymmetry (stubs, single trace vias, or inconsistent reference conditions). • Treat cables as antennas: Minimize com- mon-mode voltage at connectors. Use com- mon-mode chokes on I/O and data lines leaving the PCB. Add ferrites selected by impedance vs frequency. When possible, isolate using transformers or optocouplers. Keep noisy circuitry away from I/O connec- tors and the cable entry/exit points. • Apply shielding strategically: Shield cables properly to the chassis (low-impedance con- nection). Provide clear return paths to shunt common-mode current off the cable/shield and into the chassis. The shield should be bonded at 360°, not by pigtails. In any high-speed switching system, the active circuitry is the fundamental generator of elec- tromagnetic interference: Its rapid edge transi- tions, return current discontinuities, and para- sitic interactions create the spectral content that drives emissions. Yet it is the attached cabling that becomes the dominant radiator: an unintended, highly efficient antenna structure that couples to the common-mode currents and launches them into free space with surprising effectiveness. The circuit produces the noise energy; the cable provides the propagation mechanism, effectively transforming localized conducted noise into radi- ated energy. Like all unwanted noise, the only reli- able strategy is to suppress it at its point of origin. I-CONNECT007 Resources • Beyond Design: "Return Path Discontinuities," "Uncommon Sense-Differential Pairs," "Stackup Planning Parts 1–4," "Common Symptoms of Common Mode Radiation," all by Barry Olney • "Electromagnetic Compatibility Engineering," by Henry Ott • "Understanding Common mode noise," Pulse Electronics • "High Speed Signal Propagation," by Howard Johnson Barry Olney is manag- ing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board- level simulation. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at www. icd.com.au. To read past columns, click here. B E YO N D D E S I G N