# What LC circuit produces this response?

Discussion in 'General Electronics Chat' started by tx.x, Jun 27, 2017.

1. ### tx.x Thread Starter New Member

Jun 27, 2017
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I am trying to come up with an equivalent circuit for a structure that I'm working on. The response is shown below:

The modelling of the structure hasn't incorporated any loss, therefore the real part of the impedance is 0. I am not a circuit expert, and would like some pointers on what kind of circuits would be capable in producing a response like this.

I was pointed to this circuit:

However, it generates a different response as shown below:

Where the imaginary impedance first increases near resonance, then decreases.

I tried a few simple LC circuit configurations but wasn't able to find one that matches my expectation, I was wondering if anyone could give me some pointers.

Thanks!

2. ### crutschow Expert

Mar 14, 2008
21,423
6,137
How about a simple LC series circuit.
The impedance of that reduces near resonance and then increases after resonance.

3. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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Try replacing C2 in the crystal equivalent circuit with a 3nH inductor.

4. ### tx.x Thread Starter New Member

Jun 27, 2017
14
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This is the first thing I tried and I don't think it behaves as what I observed on the graph.

5. ### tx.x Thread Starter New Member

Jun 27, 2017
14
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I have and it didn't seem to work, does it also depend on other factors (frequency, value of L and C, etc.?)

6. ### tx.x Thread Starter New Member

Jun 27, 2017
14
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The response I got is as below

7. ### tx.x Thread Starter New Member

Jun 27, 2017
14
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Response I got for a series LC circuit:

8. ### crutschow Expert

Mar 14, 2008
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Here's the LTspice simulation of an arbitrary series, ideal LC circuit.
I drove it with a current source (1Aac) so V(in) is a measure of the circuit impedance.
As you can see, below resonance, the impedance steadily drops until going to a very low value at resonance, and then steadily rises above resonance.

9. ### tx.x Thread Starter New Member

Jun 27, 2017
14
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But the *impedance* response that I am expecting is

Y axis is impedance, not current.

10. ### BR-549 AAC Fanatic!

Sep 22, 2013
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Maybe some negative resistance will give you a range of Z at resonance.

11. ### tx.x Thread Starter New Member

Jun 27, 2017
14
0
T
The Z I'm plotting is imaginary.

12. ### AlbertHall AAC Fanatic!

Jun 4, 2014
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Why do you want this response?

13. ### tx.x Thread Starter New Member

Jun 27, 2017
14
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I'm trying to map the antenna I designed to a circuit model so I could have a systematic way of analyzing it....

14. ### crutschow Expert

Mar 14, 2008
21,423
6,137
Here's the plot in of my simulation in ohms.
The difference is that yours has the ohms scale magnitude reversed in direction from mine.

Last edited: Jun 27, 2017