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46 The PCB Design Magazine • November 2016 current is forced into a small width of copper. This increases trace resistance, inductance and skin-effect losses. Contrary to common belief, closely coupled pairs do not improve EMI. This is because it is the common-mode signals from the drivers—the natural imbalance (skew)— that radiates. In conclusion, to minimize radiation and crosstalk, of a differential pair, one must think explicitly about the common-mode component of the signal. It is not a matter of which is bet- ter—tight or loose coupling—but rather which scenario better avoids timing skew that creates this common-mode signal. It is imperative to determine exactly where the differential imped- ance levels-off. But without a good field solver that simulates signal coupling and flight time, you are really just taking a stab in the dark, which is not good design practice. Points to Remember • Conventional wisdom tells us that tight coupling between the signals is better than loose coupling, as it reduces undesirable cou- pling/crosstalk from aggressor signals. • On a PCB, the return current path of each trace of the pair flows directly below each trace in the reference plane. • If a differential pair is not perfectly bal- anced, then the degree of cancellation is not determined by the spacing, but rather by the common-mode balance. • Most digital drivers have poor common- mode balance and therefore differential pairs often radiate far more power in the common- mode than in the differential-mode. • For a perfectly balanced differential signal, the radiation from one trace exactly cancels the radiation from the other as they are equal and opposite. • The common-mode signal represents an average of the two signals in a pair. The radia- tion is identical on both traces and therefore it does not cancel but rather reinforces. • To minimize radiation and crosstalk, you must think explicitly about the common-mode component of the differential signal—skew cre- ates this common-mode signal. • To minimize the skew, match the electrical lengths and correct any shift immediately, after it arises, by adding length (hence delay) to the shorter trace. • Time-delay skew can also be introduced by a variation in the dielectric constant of the glass-resin composite. • The transformation from differential to common-mode also takes place on bends and non-symmetrical routing near via and pin ob- structions. • Mirror symmetry, about an axis along the interconnects, avoids mode transformation. The symmetry property preserves the signal in the differential-mode which does not radiate. • The differential impedance of a pair, levels- off at a particular coupling point. Beyond this point, the differential impedance will always be the same regardless of increased spacing. • As the traces get closer, both differential and common-mode impedances are reduced so the traces must be made narrower to compen- sate. • Narrow traces increase trace resistance, in- ductance and skin-effect losses. • Contrary to common belief, closely cou- pled pairs do not improve EMI. PCBDESIGN References 1. Barry Olney, Beyond Design, "Differential Pair Routing." 2. Eric Bogatin, Fundamental Myth-Concep- tions. 3. Lee Ritchey, Right the First Time. 4. Yuriy Shlepnev, Analysis of differential line transition from tight to loose coupling, and practical notes on mixed-mode transformations in differential interconnects. 5. Howard Johnson, High Speed Digital De- sign. Barry Olney is Managing Director of In-Circuit Design Pty Ltd (iCD), Australia. The company is a PCB design service bureau that specializes in board-level simulation. iCD developed the iCD Stackup Planner and iCD PDN Planner software. Visit www.icd.com.au UNCOMMON SENSE