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RLC circuit not giving correct resonance

Posted 12.06.2025, 23:05 MESZ RF & Microwave Engineering Version 6.2 3 Replies

Kyle MacRobbie
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Hi, I am trying to test a circuit with one lumped port and 3 lumped elements, one for R, L and C respectively. I am using 1 mohm, 1 nF and 1 nH as my values, so I should get a resonance at 159.15 MHz, with a Q of 1000 giving a width of 0.2 MHz. But when I sweep the frequency around this, I do not see any resonance. Why might this be? Thanks.

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3 Replies Last Post 13.06.2025, 15:07 MESZ
Robert Koslover Certified Consultant
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Posted: 22 hours ago 13.06.2025, 01:09 MESZ
Updated: 22 hours ago 13.06.2025, 01:10 MESZ

You appear to be attempting to use a purely RF model to represent an RLC circuit, while expecting purely circuit-like behavior. I perceive this as an unusual thing to do. Is it, perhaps some kind of academic exercise and/or an attempted demonstration that it is possible? After all, you could instead more cleanly represent an RLC circuit with the "electric circuit" formalism in the AC/DC module (specifically, the "ec" capability in the AC/DC module.) Instead, your approach is (deliberately or not) including more couplings (parasitics) than an explicit circuit-connected set of discrete R, L, and C components, since the RF fields extend all over. And to do that right (if that is your intention) you will need to think carefully about specifying your computational boundary conditions, the size of your computational space, and the details of your mesh. The EC formalism avoids all that additional complexity. Of course, if all you want to do is model a pure circuit, you should probably use one of the many free or low-cost SPICE codes (see https://en.wikipedia.org/wiki/SPICE) available, rather than Comsol Multiphysics. I think it is fair to say that the EC capability within Comsol Multiphysics is essentially an abbreviated version of a SPICE code, set up for convenient interfacing with FE computations. If you don't have any FE computations to join to it, it would be a bit like using a cement mixer to drive yourself, alone, back and forth to work each day; there are more appropriate tools for such a simple task.

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Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
You appear to be attempting to use a purely RF model to represent an RLC circuit, while expecting purely circuit-like behavior. I perceive this as an unusual thing to do. Is it, perhaps some kind of academic exercise and/or an attempted demonstration that it is possible? After all, you could instead more cleanly represent an RLC circuit with the "electric circuit" formalism in the AC/DC module (specifically, the "ec" capability in the AC/DC module.) Instead, your approach is (deliberately or not) including more couplings (parasitics) than an explicit circuit-connected set of discrete R, L, and C components, since the RF fields extend all over. And to do that right (if that is your intention) you will need to think carefully about specifying your computational boundary conditions, the size of your computational space, and the details of your mesh. The EC formalism avoids all that additional complexity. Of course, if *all* you want to do is model a *pure circuit*, you should probably use one of the many free or low-cost SPICE codes (see https://en.wikipedia.org/wiki/SPICE) available, rather than Comsol Multiphysics. I think it is fair to say that the EC capability within Comsol Multiphysics is essentially an abbreviated version of a SPICE code, set up for convenient interfacing with FE computations. If you don't have any FE computations to join to it, it would be a bit like using a cement mixer to drive yourself, alone, back and forth to work each day; there are more appropriate tools for such a simple task.
Acculution ApS Certified Consultant
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Posted: 13 hours ago 13.06.2025, 09:49 MESZ

Remember that a lumped circuit assumes frequencies low enough that there is no variation/wave across each component. Without opening your file, I would suggest you make the comparison at reasonable wavelength relative to the size of your components.

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René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com
Remember that a lumped circuit assumes frequencies low enough that there is no variation/wave across each component. Without opening your file, I would suggest you make the comparison at reasonable wavelength relative to the size of your components.
Kyle MacRobbie
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Posted: 8 hours ago 13.06.2025, 15:07 MESZ

Thank you for the replies,

I am trying to simulate a more complex system of a helical resonator using the RF interface, and I am trying to add a capacitance to my system through a lumped element in order to see how the resonance frequency and Q factor change with the added capacitance. Both should decrease like 1/sqrt(C), and the resonance frequency was behaving like this, but the Q factor was increasing as I increased my C.

I was thinking I might be using the lumped elements incorrectly, so I wanted to create this model as a very simple case to confirm whether or not I was doing this correctly.

Thank you for the replies, I am trying to simulate a more complex system of a helical resonator using the RF interface, and I am trying to add a capacitance to my system through a lumped element in order to see how the resonance frequency and Q factor change with the added capacitance. Both should decrease like 1/sqrt(C), and the resonance frequency was behaving like this, but the Q factor was increasing as I increased my C. I was thinking I might be using the lumped elements incorrectly, so I wanted to create this model as a very simple case to confirm whether or not I was doing this correctly.

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