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Why doesn’t Zo Scan in SIwave detect impedance changes caused by a ground cutout?

When running Zo Scan in SIwave, you may see results that indicate a trace has uniform impedance, even if there’s a ground cutout in the middle of the line.

In SIwave, each trace is divided into sections. 

  • You can confirm this by selecting the trace, going to Properties → Edit Center Line — there you’ll see the start and end points of each section.
  • The Zo Scan calculation is based only on the geometric cross-section at the start and end of each section.
  • It does not account for ground cutouts, voids, or local plane discontinuities inside the section.
  • As a result, Zo Scan will still report a uniform impedance, even though the actual EM environment is disturbed by the ground cutout.

To capture these effects, the user needs to use a full-wave solver (SYZ, AC PI, or TDR Wizard) rather than relying on Zo Scan alone.