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CPW ports definition, explicit ground and union of metals

Romolo Marcelli

Hello to everybody.

I have some concerns about the correct definition of ground in a CPW configuration defined within the AWR Microwave Office AXIEM simulator.

In the past years, I received the suggestion to use differential ports, like "1" for the signal and "-1" for the lateral ground. Of course, the same for port "2" or others. Reading again a post on this Community I have seen that somebody suggests to use "1" for the signal and "2" for the lateral ports. So, is it the same or one is better than the other?

Again, when a CPW grounded (GCPW) is studied, is it sufficient to use a metal on the bottom boundary or it must be connected to the surface using via holes to have the metal surfaces at the same potential, i.e., ground?

To end, I noticed that adding pieces of metal and making the union of them does not involve a redefinition of the metal sheet as a whole, because if such a metal is close to the I/O ports of the CPW structures, I am forced in filling all the side with the "-1" port to confirm. It seems that apparently I have one metal sheet, but in fact putting a differential port where I added some metal maintains the memory of the past configuration, forcing me in extending the "-1" definition to such a ghost metal... like in the figure attached.

Parents
  • CJ202604307615

    For CPW ports, using 1 / -1 (differential reference) vs 1 / 2 (two conductors) isn’t exactly the same. The 1 / -1 setup enforces a differential excitation with a defined return, which is often more accurate for CPW structures. The 1 / 2 approach can work, but you need to be careful about how the reference is defined.

    For GCPW, just having a bottom metal isn’t always enough. In most practical cases, vias are needed to ensure the top and bottom grounds are at the same potential, especially for accurate EM behavior.

    Regarding the ghost metal issue, it sounds like AXIEM is still treating shapes as separate conductors internally. You may need to redefine the metal as a single unified object or reassign ports after geometry changes to avoid legacy references.

  • Romolo Marcelli
    in reply to CJ202604307615

    Hi, I am still studying the differences between CPW and CPWG with Microwave Office AXIEM. Actually, for CPWG I solved the simulation problems using a number of via holes physically connecting the lateral ground plane on the top of the substrate with the bottom metal, to assure a ground to the studied structure. Nevertheless, I have a few conceptual problems: (1) the presence of via holes solved for me the comparison between a semi-lumped approach simulated circuitally (where the parameter GND=1 emulates the CPWG library elements) and the electromagnetic result; but, is this only a SW problem, or experimentally, I really need the via holes? (2) If I am simulating the simple CPW, is the differential port definition enough to have a "real" result, or do I also need more specific boundary conditions to try a direct comparison with the experiment?

  • OscPn
    in reply to Romolo Marcelli
    • 1.  No, this does not appear to be a software issue; it is related to the experimental setup. The software attempts to emulate the experimental conditions.
    • 2.  The boundary conditions defined in the experimental setup should be replicated in the software. For simulation purposes, a differential port is sufficient.
Reply
  • OscPn
    in reply to Romolo Marcelli
    • 1.  No, this does not appear to be a software issue; it is related to the experimental setup. The software attempts to emulate the experimental conditions.
    • 2.  The boundary conditions defined in the experimental setup should be replicated in the software. For simulation purposes, a differential port is sufficient.
Children
  • Romolo Marcelli
    in reply to OscPn

    Thank you very much. This actually means that a CPWG probably needs a few via holes along the path to ensure the ground conditions.

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