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December 2015 • The PCB Design Magazine 41 two conductors—not one conductor. There will be a current on the top surface of the plane, and there can be a different current or no current at all on the bottom surface of the plane. Figure 4 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 and on the same side, of the plane nearest the signal, falling off in intensity, with the square of increased distance. A stripline (inner layer) re- turn current distribution is narrower with the fields more intense above and below the trace. Any voltage drop across a ground plane will excite cables terminating on the board, which causes them to radiate as dipole or monopole antennae. The amount of current needed to cause the radiation to exceed the FCC Class B emission requirements, in a one meter long an- tenna, is extremely small—in the vicinity of just a few µA. Therefore, even the smallest ground noise voltage is significant, since it only takes a few mV of potential to produce currents of this magnitude. Power and ground planes reduce the loop area and hence the inductance and the impedance, which in turn reduces the noise. Although single-sided and double-sided boards have been used successfully in unshield- ed enclosures at frequencies of 20–25MHz, these cases are the exception rather than the rule. A design of this type, also requires a high level of EMC expertise and thus is time consuming and risky to produce. Above 10MHz, multilayer beyond design PCBs with at least two plane layers should be seriously considered. Multilayer boards reduce radiated emissions by more than 10 db compared to a double-sided board—all other factors being equal. Embed- ding signals between the planes also reduces susceptibility to radiation, as well as providing ESD protection. So not only do we prevent noise from being radiated, but we also reduce the pos- sibility of being affected by an external source. The planes in a high-speed, digital board perform five crucial functions: 1. Allow the routing of controlled impedance transmission lines in both microstrip and stripline configurations. 2. Provide a reference voltage for the exchange of digital signals. 3. Distribute stable power to all logic devices. 4. Control crosstalk between switching signals. 5. Provide a shield for electromagnetic radiation on internal layers. Next month, I will look at why solid power and ground planes encompass a distributed sys- tem of surprising complexity and how we can best use planar capacitance to reduce AC im- pedance of the PDN. Points to Remember: • Inductance may be reduced by minimiz- ing the loop area enclosed by the current flow. • Double-sided boards, using a power finger layout configuration, should be avoided as they create a large loop area for the return current. PLANE CRAzY, PART 1 Figure 4: Microstrip plane return current distribution.