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Imagine you are routing a large number of single ended high speed traces through a long narrow gap on a PCB. Lets say these are ultra-high speed SD signals, so a 208MHz clock. The traces should be 50mil to match the impedance of the source and receiver.

However, as you are short on space you have to choose one of the two options:

  1. Route at the correct width for impedance matching, at a cost of reducing your inter-trace gap to 10mil (i.e. 0.2W), thus greatly increasing crosstalk
  2. Route at 10mil width to achieve a larger gap of 50mil (i.e. 5W), at a cost of a significant impedance mismatch.

Which of these two options is the lesser evil and why? If the answer is somewhere in the middle, how do you evaluate the tradeoff? Does there exist any rules of thumb for prioritising characteristic impedance vs crosstalk?

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  • \$\begingroup\$ How long is the narrow gap and what are the signal spectrums? \$\endgroup\$
    – Andy aka
    Jun 13 '20 at 14:00
  • \$\begingroup\$ Lets say the gap is 2" long, and these are signals for an ultra high speed SD card, so around 250MHz max frequency \$\endgroup\$ Jun 13 '20 at 14:04
  • \$\begingroup\$ Add that to the question please. \$\endgroup\$
    – Andy aka
    Jun 13 '20 at 14:05
  • \$\begingroup\$ Series or parallel termination resistor? Is that a square (ish) wave of 250 MHz? \$\endgroup\$
    – Andy aka
    Jun 13 '20 at 14:08
  • \$\begingroup\$ Why not just use 10 mil all the way and a higher value series resistor? \$\endgroup\$
    – Andy aka
    Jun 13 '20 at 14:30
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Option c: use a thinner dielectric layer so that a 10 mil trace matches 50 ohms.

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  • \$\begingroup\$ You could achieve this by gluing an appropriately sized copper clad board on top of the trace(s) and grounding the copper in a few places. Make the added board thin. \$\endgroup\$
    – Andy aka
    Jun 13 '20 at 14:27

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