Design007 Magazine

PCBD-Nov2014

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40 The PCB Design Magazine • November 2014 by Barry Olney iN-CirCuiT DesigN PTY lTD BEYOND DESIGN Signal Integrity, Part 2 column In Part 1 of my signal integrity series, I ex- amined how advanced IC fabrication tech- niques have created havoc with signal quality and radiated emissions. This month's column will cover the effects of crosstalk, timing and skew on signal quality. Crosstalk Crosstalk is the unintentional electromag- netic coupling between traces on a PCB. But crosstalk can also be induced in the return path, which often gets overlooked. Figure 1 shows the crosstalk associated with two parallel trace seg- ments on the outer (microstrip) layer of a PCB. The red lines represent the magnetic field that couples voltage inductively to the nearby trace and also radiates electromagnetic emis- sions. The blue lines are electric fields that ca- pacitively couple current into the nearby trace and are somewhat absorbed by the plane but still tend to radiate noise outward. Crosstalk can be coupled trace-to-trace, on the same layer, or can be broadside coupled by traces on adjacent layers. The coupling is three- dimensional. Broadside coupling is difficult to spot, because generally we look for trace clear- ances on the same layer when evaluating cross- talk, but a simulator will pick this up. Traces routed in parallel and broadside cause greater amounts of crosstalk than those routed side by Figure 1: Crosstalk on the outer (microstrip) layer.

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