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Using Transmission Lines in SPICE circuits

Posted 14 lug 2014, 11:43 GMT-4 Electromagnetics, Low-Frequency Electromagnetics Version 4.4 6 Replies

Sam Chadwick
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I am building a model in the RF Module with an external drive circuit that is needed to provide the excitation, which is modelled using the Electrical Circuit Interface. To model the circuit accurately I need to be able to represent the properties of various lengths of coaxial cable that exist in the drive circuit. In any dedicated SPICE software this would be easily done using a Transmission Line SPICE component. However, rather annoyingly, COMSOL doesn't support transmission lines in the SPICE circuit interface!

To get around this, I have implemented a lumped-element transmission line subcircuit which can be repeated throughout the circuit. This sub-circuit consists of lumped values per unit length of the inductance, capacitance and resistance of a unit length of cable. These can then be joined up in series to create an effective (but imperfect) transmission line. The trouble with this approach is that I end up having dozens of extra circuit components which is really slowing the model down quite badly.

Does anyone have any better suggestions as to how this can be modelled accurately, and simply?

If anyone from COMSOL is reading this: Please can you add in Transmission Lines as a basic circuit component in the electrical circuits interface in the next release? It seems that the interface is rather lacking without it. Thanks!
6 Replies Last Post 18 lug 2014, 14:05 GMT-4
Dennis Brown
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Posted: 1 decade ago 16 lug 2014, 09:41 GMT-4
You are probably doing this the best way within the limited SPICE environment that comsol presents. There is the option of exporting your simulation as an s-parameter file from comsol and importing into something like Ansoft Designer SV (student version from a long time ago). In this s/w you can do TL's more effectively. This works ok for linear but if doing transient it is much more difficult. You would need the full version of ansoft designer or similar since it has the hooks built in for doing transient analysis on linear devices. Ansoft Designer SV is strictly linear.

Not only does comsol not have transmission lines it also does not have the K statement for coupling inductors. A program that is truly limiting itself.

I was going to suggest using the ABCD matrix of a transmission line but you would have to come up with a way to implement within comsol. This may get ugly.
You are probably doing this the best way within the limited SPICE environment that comsol presents. There is the option of exporting your simulation as an s-parameter file from comsol and importing into something like Ansoft Designer SV (student version from a long time ago). In this s/w you can do TL's more effectively. This works ok for linear but if doing transient it is much more difficult. You would need the full version of ansoft designer or similar since it has the hooks built in for doing transient analysis on linear devices. Ansoft Designer SV is strictly linear. Not only does comsol not have transmission lines it also does not have the K statement for coupling inductors. A program that is truly limiting itself. I was going to suggest using the ABCD matrix of a transmission line but you would have to come up with a way to implement within comsol. This may get ugly.
Sam Chadwick
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Posted: 1 decade ago 16 lug 2014, 11:31 GMT-4
Hi Dennis,

Thanks for your response. I had feared that that might be the case! It is a transient model and also involves some non-linear physics and so I don't think your suggestion would work for me. I guess I could have a play with some alternative circuit arrangements to reduce the component count, but other than that I'm a bit stuck.

I considered actually modelling the physical cables in the COMSOL model, and putting the input ports at the opposite end of the cable, but that won't work as I am working with a 2D axisymmetric model and so the input ports would need to lie along the r=0 axis, which they currently do not. Besides, the extra mesh count for adding in physical cables would probably slow the model down more than the complicated SPICE circuit currently does!
Hi Dennis, Thanks for your response. I had feared that that might be the case! It is a transient model and also involves some non-linear physics and so I don't think your suggestion would work for me. I guess I could have a play with some alternative circuit arrangements to reduce the component count, but other than that I'm a bit stuck. I considered actually modelling the physical cables in the COMSOL model, and putting the input ports at the opposite end of the cable, but that won't work as I am working with a 2D axisymmetric model and so the input ports would need to lie along the r=0 axis, which they currently do not. Besides, the extra mesh count for adding in physical cables would probably slow the model down more than the complicated SPICE circuit currently does!
Andrea Ferrario COMSOL Employee

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Posted: 1 decade ago 18 lug 2014, 03:35 GMT-4
Dear Sam,

The Electrical Circuits physics interface does not provide a transmission line functionality, however, the RF Module contains a Transmission Line physics interface that you may find useful. This interface models transmission lines on 1D geometries or on edges and can be connected to an Electrical Circuit physics interface using a Lumped Port feature in the Transmission Line interface and a External I vs. U feature in the Electrical Circuit interface.

--
Andrea Ferrario
Electromagnetics Group
COMSOL AB
Dear Sam, The Electrical Circuits physics interface does not provide a transmission line functionality, however, the RF Module contains a Transmission Line physics interface that you may find useful. This interface models transmission lines on 1D geometries or on edges and can be connected to an Electrical Circuit physics interface using a Lumped Port feature in the Transmission Line interface and a External I vs. U feature in the Electrical Circuit interface. -- Andrea Ferrario Electromagnetics Group COMSOL AB
Sam Chadwick
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Posted: 1 decade ago 18 lug 2014, 07:17 GMT-4
Hi

Thanks for your reply. the Transmission Lines interface appears to be limited to Eigenfrequency and Frequency Domain models only, and since I am working with a Transient model, I don't have the interface available to me within the physics options.

Is there any way I can incorporate it in a transient model? Or are there any other useful tools?
Hi Thanks for your reply. the Transmission Lines interface appears to be limited to Eigenfrequency and Frequency Domain models only, and since I am working with a Transient model, I don't have the interface available to me within the physics options. Is there any way I can incorporate it in a transient model? Or are there any other useful tools?
Dennis Brown
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Posted: 1 decade ago 18 lug 2014, 09:03 GMT-4
It does look like the TL interface does not support transient analysis.

I was not even aware of the TL interfance until it was mentioned. Funny thing is that, on my workstation, the interface shows up in ver4.3b but does not show up in the selection menu the latest version of 4.4.
It does look like the TL interface does not support transient analysis. I was not even aware of the TL interfance until it was mentioned. Funny thing is that, on my workstation, the interface shows up in ver4.3b but does not show up in the selection menu the latest version of 4.4.
Dennis Brown
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Posted: 1 decade ago 18 lug 2014, 14:05 GMT-4
Here is an addendum to my last reply. Sam said he was doing a simulation using an 2D axisymmetric (2DA) geometry. When I stated previously that I could not find the TL interface on my workstation, there is a reason. When I select either 1D, 2D, or 3D the TL interface shows in the physics choices. When attempting a 2DA it does not show up. I would have to say that the TL interface is not available in 2DA simulations. Even if you wanted to include the TL interface, it does not seem to work in 2DA sims. You would need to change to 1D, 2D, or 3D.
Here is an addendum to my last reply. Sam said he was doing a simulation using an 2D axisymmetric (2DA) geometry. When I stated previously that I could not find the TL interface on my workstation, there is a reason. When I select either 1D, 2D, or 3D the TL interface shows in the physics choices. When attempting a 2DA it does not show up. I would have to say that the TL interface is not available in 2DA simulations. Even if you wanted to include the TL interface, it does not seem to work in 2DA sims. You would need to change to 1D, 2D, or 3D.

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