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MAY 2021 I DESIGN007 MAGAZINE 25 the return path is undefined, then the elec- tromagnetic field will "leak" throughout the dielectric and cause common mode currents to flow all over the board. e uncontrolled field will also cause cross-coupling of clocks or other high-speed signals to dozens of other cir- cuit traces within that same dielectric through coupling to vias within the dielectric layer. e resulting common mode currents will tend to couple to "antenna-like structures," such as I/O cables or slots/apertures in shielded enclo- sures, resulting in EMI. Circuit Board Stack-Ups Most of us were taught the "circuit theory" point of view and it is important when we visu- alize how return currents want to flow back to the source. However, we also need to consider the fact that the energy of the signal is not only the current flow, but an electromagnetic wave front moving through the dielectric, or a "field theory" point of view. Keeping these two con- cepts in mind just reinforces the importance of designing transmission lines (signal trace with return path directly adjacent), rather than just course, this conduction current cannot flow through the PCB dielectric, but the charge at the wave front repels a like charge on the return plane, which "appears" as if current is flow- ing. is is the same principle where capaci- tors appear to "pass" AC current, and Maxwell called this effect "displacement current." e signal's wavefront travels at some frac- tion of the speed of light, as determined by the dielectric constant of the material, while the conduction current is comprised of a high density of free electrons moving at about 1 cm/ second. e actual physical mechanism of near light speed propagation is due to a "kink" in the E-field, which propagates along the mol- ecules of copper. e important thing is that this combination of conduction and displace- ment current must have an uninterrupted path back to the source. A high electric field is generated by high fre- quency digital signals occurring between the microstrip and return plane (or trace). If the return path is broken, the electric field will "latch on" to the next closest metal, which will not likely be the return path you want. When Figure 2: A digital signal propagating along a microstrip with currents shown. (Image source: Eric Bogatin)

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