l impedance vs common mode impedance

Dear community,
can someone explain to me some misunderstandings. There is a differencial
pair in a PCB. Simulation returns: Impedance 65.5 ohm, differencial
impedance 98 ohm, common mode impedance 82 ohm..I wonder what is the
difference between 'impedance' and 'common mode impedance'? Just 'impedance
is (as I see it) the impedance of a trace, if that trace was alone.Well.But
what common mode impedance is? Any links or tips would be appreciated.
Slav.


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