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18 DESIGN007 MAGAZINE I MARCH 2023 fringing fields of the PCB edge. is is a good reason to embed all critical signals. Both electric and magnetic field coupling can occur on PCBs; however, crosstalk usu- ally comes from electric field-based capacitive coupling. Trace segments running in parallel on the same layer or running vertically (broad- side) between two layers are extremely suscep- tible to crosstalk. Parallel traces act as capaci- tor plates separated by a dielectric. If there is a change in voltage, it will alter the electric field and generate a displacement current leading to crosstalk. Mutual capacitance between the two lines couples energy in both directions on the victim trace (Figure 2a). Inductive coupling depends on the magni- tude of the magnetic field surrounding the vic- tim trace. e magnetic field change generated by the aggressor signal surrounding the victim trace results in a changing conduction current. Mutual inductance between the lines couples energy onto the victim trace segment in the backward direction (Figure 2b). ough both mutual capacitance and mutual inductance play a role in crosstalk, there are two regimes to consider. When the return path is a wide, uniform plane, as is the case for most coupled transmission lines in a mul- tilayer PCB, the capacitively coupled energy and inductively coupled energy are of the same order of magnitude. However, when the return path is a single lead in a package, or a single pin in a connector, the inductively coupled energy is much larger than the capacitively coupled energy. In this case, the noise behavior is domi- nated by the inductively coupled energy. e noise on the victim line is driven by a dI/dt in the aggressor net, which happens at the rising and falling edges of the signal when the driver switches. Electromagnetic fields have direction and polarity. Forward (or far-end crosstalk, FEXT) and backward (or near-end crosstalk, NEXT) refer to the direction the crosstalk travels along a victim transmission line. Forward crosstalk propagates in the same direction as the aggres- sor signal, while backward crosstalk propa- gates in the opposite direction. • FEXT = (Capacitive coupling – Inductive coupling)/2 • NEXT = (Capacitive coupling + Inductive coupling)/4 In an outer layer microstrip configuration, the mutual capacitive coupling between adja- cent traces is generally weaker than the mutu- Figure 2: a) Capacitive coupling; b) Inductive coupling. (Source: Intel)