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14 The PCB Design Magazine • April 2014 any point on the board creating the connec- tion. But this does not consider the return cur- rent path which is just as important as the ac- tual trace routing for high-speed design. In a DC circuit, the return current takes the path of least resistance. But at high speeds, the return current takes the path of least induc- tance which just happens to be the reference plane (either ground or power) directly above or below the trace. The planes in a high-speed, digital board perform four crucial functions: 1. Provide a reference voltage for the exchange of digital signals 2. Distribute stable power to all logic devices 3. Control crosstalk between switching signals 4. Provide a shield for electromagnetic radiation on internal layers Figure 1, illustrates the cross-section on a microstrip (outer layer) trace and its associated plane return current distribution (red). Where the electric fields (blue) are more tightly coupled to the plane—directly below the trace—the re- turn current also exhibits tighter coupling. But where the field spreads out from the trace, the larger loop area, between the signal and the re- turn current path, increases the inductance. Re- turn current tends to couple to the signal con- ductor, falling off in intensity, with the square of increased distance. A stripline (inner layer) return current distribution is narrower with the fields more intense above and below the trace. Because of the skin affect, the high-frequen- cy fields cannot penetrate the plane and so the reference plane return currents will exist where the electric field lines terminate on the adjacent plane. Magnetic fields, which are not illustrat- ed, circle the trace and radiate outward. For a microstrip trace the return current density J(x) is given by: equation 1 where: • I is the total loop current • x is the horizontal distance out from the centre of the trace • w is the width of the trace and • h is the distance of the trace from the plane The current density will be the same regard- less of the frequency. The only constraint is that the frequency is high enough so that the resis- tance of the plane is negligible, compared with the inductive reactance. Typically, this occurs at frequencies above a few hundred kilohertz, which means basically any digital PCB. Crosstalk between adjacent traces (edge- coupled) is the result of the interaction of these fields. equation 2 Equation 2 shows that the crosstalk between adjacent microstrip traces is proportional to the feature MyTHBuSTING: THERE ARE NO ONE-WAy TRIPS! continues Figure 1: Microstrip plane return current distribution.