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I have designed a 2-layer PCB with Altium Designer. I placed an impedance rule of 50 Ohm in trace width rule section. When I run the design rule check, there is no impedance error.

However, when I run signal integrity and calculate impedance, the values around 250 Ohm is displayed.

Does anybody know the problem and its solution?

Please consider the screen shots of the Altium Designer:

layer stack

rules

rule check

signal integrity

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The issue you are having has two reasons

  1. Altium requires a power/ground plane defined as a reference. Unfortunately Altium is not capable of calculating the impedance on a 2 layer board with a copper pour as a reference. See this whitepaper
  2. The stackup and track width you have used will not give you 50 Ohms unless you use track widths of 1.5mm.

Solution: make it a 4 layer board, or use a thinner board thickness

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