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Ten Most-Important EMC Problems/Issues and How to Correct Them:
7. Crosstalk and Ground Planes



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Crosstalk

Frequently occurs due to a self-compatibility problem that happen when noisy source currents are near sensitive analog, or low-level digital signals.

Solutions:

  • Route power and signals in separate cables -- and keep those cables several inches from each other.
  • Use shielded cables.
  • In the case of circuit trace couplings, use additional separation or grounded “guard” traces between source and receptor traces.

Crosstalk found in Solid Ground Planes

Crosstalk between two conductors depends on their mutual inductance and capacitance. Often, inductive crosstalk is dominant.


Above: Cross-section of two traces showing crosstalk. Crosstalk = K / (1 + (D / H)2)

Minimizing Crosstalk via a Guard Trace


Above: Three different microstrip structures tested for noise at the receiver of the victim line.

Use the “3W” Rule During Layout

  • Clocks and periodic signals have a highly-probable chance of creating crosstalk in other traces.
  • Reduce crosstalk effects via the “3W” rule: make sure all critical traces are “buffered” by at least 3 trace-widths surrounding each potential source and victim trace.
  • Increase trace spacing from 1W to 3W; this will decrease far-end crosstalk by 65%.

Differential Pair Traces and the “3W” Rule

Parallel differnetial pair routing and the 3-w rule.

Most EMI/EMC engineers consider 3-5W the absolute minimum spacing for any critical trace to board edge spacing.

       
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Updated: Saturday, 2007-09-29 1:02 PST