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I have a small problem with my LTspice simulation...

When you have a system that contains multiple capacitors, inductors and resistors, how do you know, when you measure its equivalent impedance Z using an .ac simulation in LTspice (for example), what components resonate (which L with which C). To give you guys a context, I have [this][2] schematic below, that has the same resonance frequency (but not the same amplitude though) to same schematic but with different values of the parameters: R12, C12, R13, C13, Rm1, Cm1, Rm2, Cm2, Rm3, Cm3. Does this seem reasonable to you guys? or do you think I am doing something wrong somewhere?

Schematic

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    \$\begingroup\$ Read this question and see if it clarifies things for you. \$\endgroup\$
    – relayman357
    Commented Nov 11, 2020 at 18:53
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    \$\begingroup\$ A circuit in general doesn't have a equivalent impedance – a two-port thing has, so you must define which two points you're referring to, @Wallflower. Then, in general, a circuit doesn't necessarily have a resonance frequency at all, or only one. There's nothing that you describe which we can attribute the term "reasonable" to, so I'm as confused as Andy about your question. \$\endgroup\$
    – Marcus Müller
    Commented Nov 11, 2020 at 18:56
  • \$\begingroup\$ @relayman357 thank you. The impedance was computed using Ohm's law : Z=V1/I(V1). Why can't a circuit have a resonance frequency when it has inductors and capacitors in it? They are the two components that resonate, are they not? \$\endgroup\$
    – Wallflower
    Commented Nov 11, 2020 at 18:59
  • \$\begingroup\$ The circuit may have a resonance frequency, or maybe not (at least not clearly). It may have multiple (see Marcus' comment above). Are you calculating your Z at one frequency? You need to run a frequency sweep if you are looking for resonances. \$\endgroup\$
    – relayman357
    Commented Nov 11, 2020 at 19:15
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    \$\begingroup\$ Your four sections are missing grounds. Maybe that's implied. \$\endgroup\$
    – Reinderien
    Commented Nov 11, 2020 at 20:16

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You cannot determine from measurements at just one port of the circuit what the internal structure of the circuit resonances are....that is, what the current pattern is within the circuit. If you can identify a resonance from the behavior at one port with a frequency sweep, you could measure or simulate the currents or voltages in the components within the circuit when driven at that frequency to determine which components are 'participating' in that resonance. By 'participating' I mean that for the particular resonance, the component has current running through it.

If a component has no current through it when the circuit is driven at a resonant frequency, then that component is not participating in that particular resonance. A resonance could involve just a subset of the components, and so a resonant frequency could be completely insensitive to the values of some of the components in the circuit (the non-participating ones).

There is another way to analyze resonant circuits like this with an ac mesh analysis, which involves finding eigenfrequencies of a set of matrix equations, but this is a rather different set of tools than what you are using.

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  • \$\begingroup\$ I will try looking at the current in the components of the circuit to determine who participate. Thank you @rpm2718 \$\endgroup\$
    – Wallflower
    Commented Nov 11, 2020 at 20:26
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    \$\begingroup\$ OK, glad to have helped. I might also note that a peak in the impedance magnitude at one port does not necessarily imply a resonance. To identify something as a resonance from a one-port frequency sweep with reasonable confidence, the resonance would have to be pretty high Q \$\endgroup\$
    – rpm2718
    Commented Nov 11, 2020 at 20:37

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