Robert Koslover
Certified Consultant
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Posted:
7 years ago
09.07.2017, 22:23 GMT-4
My recommendation: Don't use a lumped port for this. With a lumped port, the code generally assumes you already know the impedance, and that you are specifying things like voltage or current, and that the detailed field distribution in the port region is of minimal interest. But that is NOT your case, since the detailed field distribution at the port is what determines the impedance, Instead, you need to use a "numeric" port. This (when done right) separates the problem into two parts. First, Comsol Multiphysics computes (i.e., numerically solves) for the fields in the port region (a 2D subset of your 3D problem) then applies those fields to the transmission line for the 3D problem. I think if you hunt around you may find an example in the library files. For what it is worth, this trick used to be a lot more difficult to do correctly in older versions of Comsol Multiphysics. In the newer versions, once you set up the port right, you can get the software to set up the solver sequence for you, which is a big help. OK, now I realize that the above notes aren't a detailed step-by-step explanation, but I hope this will at least help you to get going in the right direction. Good luck.
My recommendation: Don't use a lumped port for this. With a lumped port, the code generally assumes you already know the impedance, and that you are specifying things like voltage or current, and that the detailed field distribution in the port region is of minimal interest. But that is NOT your case, since the detailed field distribution at the port is what determines the impedance, Instead, you need to use a "numeric" port. This (when done right) separates the problem into two parts. First, Comsol Multiphysics computes (i.e., numerically solves) for the fields in the port region (a 2D subset of your 3D problem) then applies those fields to the transmission line for the 3D problem. I think if you hunt around you may find an example in the library files. For what it is worth, this trick used to be a lot more difficult to do correctly in older versions of Comsol Multiphysics. In the newer versions, once you set up the port right, you can get the software to set up the solver sequence for you, which is a big help. OK, now I realize that the above notes aren't a detailed step-by-step explanation, but I hope this will at least help you to get going in the right direction. Good luck.
Dave Greve
Certified Consultant
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Posted:
7 years ago
10.07.2017, 14:33 GMT-4
Are you using the free-space lambda in your equation?
That expression only works if you use the guide wavelength, which will be different (by roughly a factor of 1/sqrt(epsilon).
D.W. Greve
DWGreve Consulting
Are you using the free-space lambda in your equation?
That expression only works if you use the guide wavelength, which will be different (by roughly a factor of 1/sqrt(epsilon).
D.W. Greve
DWGreve Consulting
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Posted:
7 years ago
10.07.2017, 14:53 GMT-4
In the equation I have shown above, yes I am using the free-space lambda of
![](data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAADUAAAAWCAQAAADK1xxjAAAAAmJLR0QA/4ePzL8AAAAJcEhZcwAAAHgAAAB4AJ31WmAAAAAHdElNRQfoCB8AGyCckdQVAAABpklEQVRIx72W4XXrIAyFP73TBViBjuCOwBuBjpAV4hGcEewRkhHqEeoRygqMoPfDmIj2tWmbnMh/BAgu90roWJR72Z+7IbVQ4t8vS/dx7moo8TIwNjBRBhydjLeBe1jvTiQTyA3DQR8BJHHk6QZYWj9eeDGjkaH6r0Tl2u/zsojGz4QbCfhfc8xmVLMlgaB98fdMmtc50F46Ag60F0fEAyddLkBJy2IxrKKJGfQAQKe9vAkkPYAcZSAzaZbIkcfLrFrrqhc4VG8GkMi88tZpk1ufimdq97tPOBUmHl+F7VjFybpIgE1WAqd3EV9CJcAZTqkek/TsLwA6nxmCuOYy80UoTY1o1BdXN0sAPdX18/0DeS0FcUROF6HABuHsQTZTsmvz1jCJJF3EbzGfQ01brYnHM1VvkymyUB6BhCKjkbXmrDIrFbh2O5CRzKQJdJZJRnocI381b5mik4DD0RNlX44NWCmfqy5B9iTddPiqleDZs8OZZjUqBHxZ9WXemRjf7Hdm9JMuxhu73/fAn4R61N75du32o0Xzpn5h8v1/C3G4O0Fda/8ADaQ/56DohJcAAAAtdEVYdGljYzpjb3B5cmlnaHQAQ29weXJpZ2h0IEFydGlmZXggU29mdHdhcmUgMjAxMQi6xbQAAAAxdEVYdGljYzpkZXNjcmlwdGlvbgBBcnRpZmV4IFNvZnR3YXJlIHNSR0IgSUNDIFByb2ZpbGUTDAGGAAAAIXRFWHRwczpIaVJlc0JvdW5kaW5nQm94ADMyeDEzKzI5MCs2MzYnZ2AIAAAAHnRFWHRwczpMZXZlbABQUy1BZG9iZS0yLjAgRVBTRi0yLjBB+TMTAAAAAElFTkSuQmCC)
. But also in the equation is
![](data:image/png;base64,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)
, which already accounts for the wavelength change in SiO2.
In the equation I have shown above, yes I am using the free-space lambda of [math] 10 \mu m [/math]. But also in the equation is [math] n =\sqrt{\epsilon_r} [/math], which already accounts for the wavelength change in SiO2.
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Posted:
7 years ago
10.07.2017, 14:55 GMT-4
Updated:
7 years ago
10.07.2017, 14:56 GMT-4
Robert,
Thanks for the reply! I have tried toying with a numerical port but keep getting an error saying "integration lines defining voltage and current are not defined." I will try to pursue this further and find some examples on line if possible.
Robert,
Thanks for the reply! I have tried toying with a numerical port but keep getting an error saying "integration lines defining voltage and current are not defined." I will try to pursue this further and find some examples on line if possible.
Dave Greve
Certified Consultant
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Posted:
7 years ago
10.07.2017, 15:56 GMT-4
OK a couple more possibilities and a suggestion.
1. Using sqrt (epsilon) is only approximate, as there would be some electric field fringing.
2. Make sure the value for epsilon is correct. Comsol 5.3 has three different things that are similar to SiO2: SiO2, glass(quartz), and silica glass. The permittivity for silica glass is wrong.
The suggestion: draw your geometry in 2D, and use Mode Analysis (only available in 2D). Not Boundary Mode Analysis. See
www.comsol.com/model/finding-the-impedance-of-a-parallel-wire-transmission-line-12403
(Your question made me realize that the way I was doing line impedance calculations in 3D was unnecessarily complex.)
OK a couple more possibilities and a suggestion.
1. Using sqrt (epsilon) is only approximate, as there would be some electric field fringing.
2. Make sure the value for epsilon is correct. Comsol 5.3 has three different things that are similar to SiO2: SiO2, glass(quartz), and silica glass. The permittivity for silica glass is wrong.
The suggestion: draw your geometry in 2D, and use Mode Analysis (only available in 2D). Not Boundary Mode Analysis. See
https://www.comsol.com/model/finding-the-impedance-of-a-parallel-wire-transmission-line-12403
(Your question made me realize that the way I was doing line impedance calculations in 3D was unnecessarily complex.)
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Posted:
7 years ago
10.07.2017, 16:23 GMT-4
Thank you. The value I'm using for epsilon is
![](data:image/png;base64,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)
, which was taken directly from the SiO2 material information in COMSOL, so I know that this is not the issue. I will try to do the 2D mode analysis but then I will still have this problem as I will eventually need to model this as a more complex 3D device.
Thank you. The value I'm using for epsilon is [math] \epsilon_r = 3.9 [/math], which was taken directly from the SiO2 material information in COMSOL, so I know that this is not the issue. I will try to do the 2D mode analysis but then I will still have this problem as I will eventually need to model this as a more complex 3D device.
Robert Koslover
Certified Consultant
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Posted:
7 years ago
10.07.2017, 17:05 GMT-4
I've attached an example of 3D stripline model that you may find helpful. In this case, the substrate is air, but you could certainly modify the model to change that. Also, I have found that integrating to find currents using the local H fields can be more accurate than using Comsol Multiphysics' computed surface current terms. Anyway, see what you think. Good luck.
I've attached an example of 3D stripline model that you may find helpful. In this case, the substrate is air, but you could certainly modify the model to change that. Also, I have found that integrating to find currents using the local H fields can be more accurate than using Comsol Multiphysics' computed surface current terms. Anyway, see what you think. Good luck.
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Posted:
7 years ago
10.07.2017, 19:15 GMT-4
Robert,
Thank you so much! I will take a look at the file you attached and see if I can work it out for there. I really appreciate it.
Robert,
Thank you so much! I will take a look at the file you attached and see if I can work it out for there. I really appreciate it.
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Posted:
7 years ago
10.07.2017, 19:20 GMT-4
Did you make this file in version 5.3? Unfortunately I'm running 5.2a and am unable to open the file you attached. Do you know of a way around this issue? Thank you for all of your help!
Did you make this file in version 5.3? Unfortunately I'm running 5.2a and am unable to open the file you attached. Do you know of a way around this issue? Thank you for all of your help!
Robert Koslover
Certified Consultant
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Posted:
7 years ago
10.07.2017, 19:56 GMT-4
Yes, I used version 5.3 (I try to always use the latest version). I don't seem to have an easy/fast way to export the file to an earlier version -- it seems I would have to rebuild it mostly from scratch.
If you have a current license/support, you should be able to update your Comsol software to the latest version. I encourage you to do that, if you can.
Meanwhile, although it is not as close a fit to your problem, you may be able to learn something from looking at the Comsol application library file for the "waveguide adapter" in regard to the use of numeric ports. Take a look at it, if you haven't already. Another one (in 2D) in the Comsol library that might help you is the "parallel_wires_impedance" model, which is listed under the "verification examples" in the Application Library.
Yes, I used version 5.3 (I try to always use the latest version). I don't seem to have an easy/fast way to export the file to an earlier version -- it seems I would have to rebuild it mostly from scratch.
If you have a current license/support, you should be able to update your Comsol software to the latest version. I encourage you to do that, if you can.
Meanwhile, although it is not as close a fit to your problem, you may be able to learn something from looking at the Comsol application library file for the "waveguide adapter" in regard to the use of numeric ports. Take a look at it, if you haven't already. Another one (in 2D) in the Comsol library that might help you is the "parallel_wires_impedance" model, which is listed under the "verification examples" in the Application Library.