HFSS Combining Reference Grounds

[ferdinnand]

Hi all,

I'm trying to model a rather ripped coax cable which has a particular region of 10mm in the middle, where the outer conductor is stripped off. In reality, both ends of the cable would be connected to the ports of a network analyser so that they'd have a single reference ground. Yet in HFSS, I cannot find a way to assign a common ground to the structure. As you can see in the picture I tried to put a perfect-E rectangle (in blue) just outside at the ends of the outer conductor but that of course inserts its own parasitic inductance, and much worse, depending on the width of the rectangle. Here is what I get as reflection from north to 1GHz;


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My question is how to create an ideal model in HFSS to connect my ground conductors at both ends of my coax without adding parasitic inductance ?


PS: The reason behind my intention to simulate such model is to find out a proper way to combine several ground conductors in one of my package design projects having lots of ports with separate grounds which have to be combined somehow. So in terms of pure imagination, what I'd like to have basically is a zero-length conducting path model if possible.

 

[ferdinnand]

bumping for any suggestions

 

[Marco_A_Llamas]

Hi,

very interesting post. I have been thinking in something similar in the past. The problem you have is that your way to connect your separate grounds is affecting your structure.


One way I found is to connect separate grounds using the outer air box. What I mean is that you can use the air box with a Perfect E boundary condition where all your wave ports grounds will be connected. This can be done taking into account that the air box has to be big enough (This boundary condition doesn't have to affect the structure and its fields) and your structure has to be non radiatiative. Then the outer Airbox acts like your NA ground where all your probes are connected without impacting your structure performance.

Let me know is that works for your model.

Good luck

 

[ferdinnand]

Thank you for your reply,

Sadly though I'd already tried using the boundary itself as a ground plane but unfortunately it turned out to be even more problematic. The main problem with such perfect E-field boundary is that I cannot get rid of phase differences along the plane of the boundary, which is not surprising as that is basically how a perfect conductor plane would act irl. Therefore the reflection then strongly depends on the size of the box and even rises above -0.1dB once the pathway of currents on the box becomes closer to lambda/4. Having that, I have also tried increasing the size of the box to a point where the path would be lambda/2 but again no success, assuming the parasitic capacitance of the box was causing the problem there.

So then I believe there are no hypothetical elements or boundary methods in HFSS to get rid of phase difference, it seems I have to replace my ports in my design by means of attaching U-shaped coax probes where I'd like to measure and connecting them all to the body of my package.

 

[Marco_A_Llamas]

Hi Ferdinand some questions:

First of all what do you mean by no success? What result do you expect to obtain?

Second question: What do you mean when you talk about that the "parasitic capacitance of the box was causing the problem there"

 

[ferdinnand]

Hi,

As for the first, the reflections have strong dependency on the size of the box. And of course it also depends on the wavelength, thus I rather prefer to look for a broadband solution. Generally speaking, the result I expect would be such that there should be much less change in the reflections as I alter the physical dimensions of the structure which I use to connect the grounds.

The second one is rather my inference, the structure as you know has parts in the middle where the wave is no longer fully guided and the E-field can be directed into free space. I suspect that introducing a perfect conductor plane surrounding that particular area affects the capacitive reactance as it can interact with those fields.