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108 I-CONNECT007 MAGAZINE I FEBRUARY 2026 provides little to no benefit in lowering radiation. Figure 4 illustrates the simulated common-mode return current in the reference plane beneath an imbalanced differential pair. Note that the return path of the non-inverting and inverting differen- tial signal is not in the opposite pair, but rather the return path for both signals is in the reference plane. Closely coupled differential signals operate primarily in differential mode, with only minor common-mode radiation caused by any imbal- ance between the traces. However, if the traces are spaced far enough apart that coupling is lost, and they behave as two independent single-ended lines. In that case, a 100-ohm differential pair effec- tively becomes two 50 ohm single-ended signals, which is perfectly acceptable as long as the loop area remains small and the impedance stays consistent along the entire length of the routing. To make matters worse, once cables are connected to the PCB, they inherit this common- mode ground potential and effectively become monopole antennas. Remarkably, as little as 3 µA of common-mode current on a 1 m cable at 100 MHz is enough to fail an FCC Class B EMC test. While differential-mode radiation can be managed through careful stackup design and routing, common-mode radiation is far harder to predict and control because it is unintentionally engi- neered into the system. Unfortunately, schematics do not reveal the often-unexpected current return paths that are critical to understanding signal integrity, crosstalk, and radiated emissions. Unrelated power planes should never extend into the ground region of the I/O connector area. These planes often carry high-frequency switch- ing noise, and if they encroach on the I/O zone, that noise can readily couple into the I/O signals and their reference ground. To prevent this, the I/O ground must connect to the enclosure or chas- sis ground at a single, well-defined, low-imped- ance point. This minimizes noise injection, controls return-current paths, and maintains signal integrity at the interface boundary. To effectively control common-mode radia- tion, the priority is to reduce the common-mode ground voltage that drives unintended antennas at the source. PDN noise is a dominant contribu- tor to radiated emissions. Suppressing this noise requires preventing it from propagating out of the processor and into the power and ground planes, while designing a PDN whose AC imped- ance stays below the target impedance across the full bandwidth. Achieving a low-impedance PDN involves minimizing the spacing between power and ground planes, minimizing the loop area and using low-inductance, low-impedance decoupling capacitors. Plane cavity resonance can also gener- ate standing waves within the cavity, which in turn amplify the overall resonance. Because these interactions are complex and highly frequency- dependent, using a PDN planning tool is strongly recommended. Good grounding also helps suppress noise by giving common-mode currents a low-impedance path back to ground. Incorporating multiple ground planes in the stackup is one of the most effective ways to achieve this. Just as important is maintain- ing those planes as solid, uninterrupted surfaces— slots or splits can severely disrupt return paths and should be avoided. When the return path for a common-mode current is physically separated from the signal path, the resulting large loop area leads to radi- ation. But if the return path is engineered to stay close to the source current, the loop area remains small, and radiation is minimized. In other words, Figure 4: Coupled microstrip differential pair. (Source: Ansoft) Figure 5: Common mode signal return path. B E YO N D D E S I G N