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62 DESIGN007 MAGAZINE I FEBRUARY 2021 tends to radiate from the outer layers of a mul- tilayer PCB whereas stripline confines the EM fields between the planes. Crosstalk also depends on the load which may vary considerably when driving banks of memory modules, for example. Keep in mind that the total crosstalk on each victim trace is the total crosstalk from each of several nearby aggressors, all of which sum up to produce the maximum value. Both forward and reverse crosstalk can be arbitrarily reduced by separating the aggressor(s) from the victim trace(s) or by reducing the height of the dielectric above/ below the planes. e latter also requires a reduction in trace width to maintain the impedance. If real estate is a premium, as it generally is on dense, high-speed designs, then routing on the inner layers and avoiding broad- side coupling may be good solutions. Key Points: • Crosstalk is three dimensional and is dependent on the signal trace separation, the trace to plane(s) separation, parallel segment length, the transmission line load, and the technology employed • Crosstalk also varies depending on the physical stackup configuration • Close coupling is good for maintaining impedance, rejecting noise and avoiding slew, but not good for crosstalk • Far-end (FEXT) and near-end (NEXT) crosstalk refer to where the crosstalk is measured—at the load or the receiver, respectively • A unique property of the stripline configuration is that the ratio of mutual capacitance equals that of the mutual inductance, which cancels out the forward crosstalk component Figure 4: Crosstalk vs. trace spacing (edge coupled).