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Why the Zdiff of coupled line changes with length?

Question asked by DEPAOLA on Sep 24, 2006
Latest reply on Sep 24, 2006 by ALEPEREZ
I am trying to design two copled lines using momentum. My substrate is made of silicon with oxide on it and I am using the thick metal expansion. The ports I am using are 4 (two on each side of the coupled line) and I defined them as single with 50 ohm impedance. In order to find the differential impedance, I am using a schematic with two s-parameters ports connected in a differential way and I am sweeping the value of the port impedance in order to find the minimum db(S11). Is it normal that, by changing only the length of the line and keeping the width and gap constant, the value of Zx that minimizes  S11 changes (not slightly but, let's say from 100 Ohm to 80 Ohm)?? With a TLINE component, the value of characteristic impedance is constant, no matter which length is used (as it should be I guess).
Please help me...