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My Question
I have an admittance in complex form for a circuit.
Is it possible to know what components are in the circuit based on the admittance?
How do I calculate it?

August Jelemson
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2 Answers2

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Is it possible to know what components are in the circuit based on the admittance?

Unfortunately not because a single spectral point admittance value tells you nothing about what that admittance changes to at other frequencies AND you need a full spectrum of admittance (or impedance) values to make that determination. Even then, it won't tell you what the exact "black-box" circuit is; it only gives you an equivalent model.

Andy aka
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  • okay, but if i know that i only have 2 components in the circuit, could it then be determined based on the admittance?, could i assume that the real part of the admittance corresponds to a resistor and then based on if the complex part is negative say that its a conductor? – August Jelemson Mar 31 '21 at 10:03
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    Sure, if you have two passive components inside the "black box" then there's a good chance that a couple of spectral points will deliver the result. If you only have one spectral value then you would not be able to distinguish if the components were in series or in parallel because, at any single frequency, a series R and L is equivalent to a different R in parallel with a different value L. – Andy aka Mar 31 '21 at 10:10
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    @AugustJelemson let's assume you have 2 components connected to make a 1 port circuit in a box. You see the admittance in a certain fixed frequency is 0.05 - 0.01j ; both measured in mhO's. You have still no way to decide is there an inductor, a quartz crystal or a transformer making the imaginary part before you make more measurements or open the box. –  Mar 31 '21 at 13:47
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Well, the definition of susceptance is the following:

$$\text{susceptance}=\Im\left(\text{admittance}\right)=\Im\left(\frac{1}{\text{impedance}}\right)\tag1$$

In formula form:

$$\text{B}=\Im\left(\underline{\text{Y}}\right)=\Im\left(\frac{1}{\underline{\text{Z}}}\right)\tag2$$

Jan Eerland
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