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Microstrip trace Impedance with cross-hatched Ground plane

Question asked by on Nov 5, 2013
Latest reply on Nov 7, 2013 by

I have a Flex circuit made out of Polyimide (er =3.3 to 3.5), I want to confirm the impedance of single ended microstrip and differential even/odd mode traces using ADS2011 Layout.  I have loaded the Gerber files into the tool, and I can setup multiple ports.  My goal is to extract the single ended/diff impedance in the presence of a cross-hatched plane (not solid).  The flex circuit is used in a dynamic environment, and it cannot be a solid plane reference as it will fatigue it, and eventually fail over time (cracked traces etc).  I want to adjust the traces, and cross-hatch parameters to hit 90 ohms differential impedance, and 50 ohms single ended.

I will split the problem in two cases -- the differential and single ended.  I want to setup a case for the differential, and extract the impedance of the traces for even and odd mode.  I am sure this is a piece of cake in ADS, I just don't know how to set it up.  I know I can convert the impedance Z11 = 50*(1+S(1,1))/(1-S(1,1)).  From here how do I get odd and even mode?  How do I parametrize the cross hatch so I can vary the some parameters and look at the impedance changes?

I would really appreciate it any help.  I have dug up my old books and I recapped the impedance for even=Z11/2 odd=2*Z11.   However, I just need to know how many ports I need to put on my traces. For the differential case, I imagine this is a two port measurement, but what about the ground reference?  I would put the following ports P1,P2 to diff pair one end. P3,P4 to the other end, what about ground? Is it just P5?  Do I then export this to schematic, and add ground since I can't add ground in Layout?

Any help would be greatly appreciated.  This is a classical case I have gotten around experimentally and through FCC testing, but I don't have a simulation model to predict this without iterating the flex.