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106 I-CONNECT007 MAGAZINE I FEBRUARY 2026 If I 1 and I 2 are the currents in the two conductors, then: Differential Mode Current (I DM ) = (I 1 - I 2 )/2 Common Mode Current (I CM ) = (I 1 + I 2 )/2 Differential-mode radiation is an inherent part of normal circuit operation, arising from currents flow- ing through the return-path loop formed by PCB conductors: both traces and their associated return paths (Figure 2). Microstrip loops on outer layers can behave like small antennas, primarily radiat- ing magnetic fields, while stripline loops on inner layers radiate only through the fringing fields at the board edges. Although these signal loops are essential for proper functionality, their physical size and loop area must be carefully controlled during design to keep radiated emissions to a minimum. The most critical radiation-producing loops are those carrying the high-frequency periodic signals. In synchronous systems, the clock, being a contin- uous stream of repetitive pulses, is typically the dominant emitter. Clock routing should therefore be prioritized, with every effort made to minimize loop area. This includes keeping the clock trace as short as possible and reducing the number of layer-transitions. On multilayer PCBs, clock lines are best routed as striplines on inner layers adjacent to a solid reference plane, which helps contain fields and reduce radiation. Tight spacing between the clock trace and its return plane further increases coupling and shrinks the loop area. Additionally, to avoid coupling clock energy into external cables, clock circuitry should be placed well away from I/O connectors and cable interfaces. Data and address buses, along with their command and control lines, are the next major contributors to emissions. Even with proper termi- nation, these interconnects can carry significant bursts of electromagnetic energy, and the resulting radiation scales with the magnitude of that energy. Transient power-supply activity is another key source of differential-mode emissions. Although the associated loops may be physically small, they can concentrate intense switching electromagnetic energy, making them disproportionately strong radiators. Differential-mode radiation increases with the square of frequency, and it can be managed through several design strategies: lowering the power distribution network (PDN) impedance below the target value, minimizing loop area, using differential signaling to cancel fields, and apply- ing clock dithering. Spreading the emission energy across a wider frequency band also reduces peak radiation levels. In practice, spread-spectrum clocking can cut radiated emissions by as much as 15 dB. When a differential pair is well balanced, tight coupling helps achieve strong field cancellation. But if the pair is even slightly imbalanced (Figure 3), the effectiveness of cancellation is no longer governed by trace spacing. It is dictated by the pair's common-mode balance. Any imbalance in the routing must be corrected at the point where it occurs to preserve field cancellation. Also, due to the inherently poor common-mode balance of most digital drivers, differential pairs frequently emit much stronger common-mode radi- ation than differential-mode radiation. In these cases, reducing the spacing between the traces Figure 1: Differential and common-mode fields. Figure 2: Differential and common-mode currents. Figure 3: Differential mode signals can be converted to com- mon mode by displacement current. B E YO N D D E S I G N