Through inductive or mutual coupling, the flux in a circuit loop on one layer can easily link a second circuit loop on an adjacent layer thus inducing unwanted noise voltages in the second circuit loop. The voltage induced is at a maximum when the loops are positioned directly on top of each other on the adjacent layers. Whenever there is current flow in a loop, there is also flux produced around the traces forming the loop. Since the flux is directly proportional to the current in source loop and the mutual inductance between the two loops, the coupled noise signal is going to increase with increasing current in the source loop. In other words, the greater the current in the source loop, the greater noise voltage will be induced in the victim loop on the adjacent layer at a given frequency where the loops on the adjacent layers overlap each other. Since the induced voltage also scales with frequency for a constant flux, the crosstalk will be much greater at the higher frequencies than at low frequencies. By routing the traces on adjacent layers so the traces on one layer are oriented perpendicular to traces on an adjacent layer,  the overlapping loops on adjacent layers will be minimized thus reducing the possibility of crosstalk by mutual coupling between the adjacent layers.

While mutual coupling can be a problem between adjacent layers, especially when higher currents are involved, the other possibility we have is capacitive coupling between adjacent layers. Capacitive coupling is more likely to occur if higher voltages are present and the victim circuit is a high impedance circuit. By routing the traces on adjacent layers so the traces on one layer are perpendicular to the traces on an adjacent layer, capacitive coupling is minimized because the overall surface area of traces which can form the "plates" of capacitors are minimized. 
 
 
 

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EMC Tip 15