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Dear kimjin,
Intro: the impedance calculation depens on track width and board stack up(Dielectric properties)
1: set the board stack up from PCB SI (Setup>Cross-section) and input the defult impedance value. this value will be defult when u need to route and connection line(this mean when u need to route any Cline the Allegro will calculate the Cline width that will give this impedance and route using it regard less U change the track width)
2: if there some tracks has 50 and other has other impedance say 75 u can Use constrain maneger to spacify which net has which impedance and let constrain maneger automatically calculate the track width that will give the impedance that u assigeened for it.
With the stack up information, you can alsu use formulas found on the web to calculate impedence. You can verify these values with the fab. house. Then set the net properties to have certain trace widths depending on desired impedence.
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