I/O Interfaces & Buses

Why are high-speed signal traces designed so that their characteristic impedance matches the transmit and receive circuits?

  1. ATo reduce trace resistance to zero and eliminate loss
  2. BTo make traces glow so they can be checked visually
  3. CTo prevent reflections at impedance discontinuities from distorting the waveform
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Answer: C. To prevent reflections at impedance discontinuities from distorting the waveform

At high speed, a trace behaves as a transmission line along which electrical waves travel. Wherever the characteristic impedance changes, at connectors, vias or mismatched terminations, part of the signal reflects back and overlaps later bits, distorting the waveform. Designers therefore tune trace width and layer spacing so that, for example, differential traces are 100 Ω, and terminate them with the same value.

  • A. Matching impedance does not remove resistive loss.
  • B. Traces do not glow.
  • C. Correct. Differential traces target values such as 100 Ω.
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