# Calculate inductance of a coil in CST

1. ## Calculate inductance of a coil in CST

How to calculate the inductance of a coil in CST ? for the attached coil

How to feed the coil in order to get the inductance ?

2. ## Re: Calculate inductance of a coil in CST

It depends what configuration you are interested in, see

If you want 1-port data with a lumped port between the ends of the turns, you should include the underpass (bridge), so that you have a clearly defined current path.

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3. ## Re: Calculate inductance of a coil in CST

Hye,
If i'm using one port between ends of turns, how to find the inductance of the coil?

4. ## Re: Calculate inductance of a coil in CST

Originally Posted by liana82
Hye,
If i'm using one port between ends of turns, how to find the inductance of the coil?
Convert S to Z-parameters
L = im{Z11} / (2*Π*f)

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5. ## Re: Calculate inductance of a coil in CST

Originally Posted by volker@muehlhaus
Convert S to Z-parameters
L = im{Z11} / (2*Π*f)
How to find the frequency response of the coil?

6. ## Re: Calculate inductance of a coil in CST

Originally Posted by liana82
How to find the frequency response of the coil?
What exactly do you mean by "frequency response" ? Which parameter ?

You get frequency dependent S and Z parameters from simulation, so the calculated L from the above equation is also a function of frequency.

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7. ## Re: Calculate inductance of a coil in CST

Originally Posted by volker@muehlhaus
What exactly do you mean by "frequency response" ? Which parameter ?

You get frequency dependent S and Z parameters from simulation, so the calculated L from the above equation is also a function of frequency.
So...the inductance will change according the frequency? and how o find the Q factor of the coil in CST?

8. ## Re: Calculate inductance of a coil in CST

Originally Posted by liana82
So...the inductance will change according the frequency?
Yes, the effective inductance will change with frequency

Originally Posted by liana82
and how o find the Q factor of the coil in CST?
Not sure how to calculate that in CST, but Q = imag{Z11} / real {Z11}, as described in my appnote linked above #2

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9. ## Re: Calculate inductance of a coil in CST

Originally Posted by volker@muehlhaus
Yes, the effective inductance will change with frequency

Not sure how to calculate that in CST, but Q = imag{Z11} / real {Z11}, as described in my appnote linked above #2
If i simulate planar square coil in frequency range 10MHz-20MHz, which simulation tool of CST i should choose? MWS or EM?Because my simulation has 10 billion mesh size! Please help.

10. ## Re: Calculate inductance of a coil in CST

I use ADS with Momentum (Method of Moments) as the preferred solver, but FEM works as well.

I never tried the time domain solver for this type of problems. Time domain is a good choice for electrially large models, but inductors are electrically small. If you don't have MoM, I suggest you use the FEM solver.

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11. ## Re: Calculate inductance of a coil in CST

Originally Posted by volker@muehlhaus
What exactly do you mean by "frequency response" ? Which parameter ?

You get frequency dependent S and Z parameters from simulation, so the calculated L from the above equation is also a function of frequency.
I mean how to get resonant frequency of the coil?

12. ## Re: Calculate inductance of a coil in CST

Originally Posted by liana82
I mean how to get resonant frequency of the coil?
I don't know what math CST offers you, but I evaluate the L(freq) curve to get the frequency where L changes from positive to negative value.

http://muehlhaus.com/wp-content/uplo...une_result.png

13. ## Re: Calculate inductance of a coil in CST

http://muehlhaus.com/wp-content/uplo...une_result.png[/QUOTE]

What software you use to get this results?How to use it?

14. ## Re: Calculate inductance of a coil in CST

Originally Posted by liana82
http://muehlhaus.com/wp-content/uplo...une_result.png
What software you use to get this results?How to use it?
This is RFIC Inductor Toolkit for ADS

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15. ## Re: Calculate inductance of a coil in CST

[QUOTE=volker@muehlhaus;1589183]Yes, the effective inductance will change with frequency

If the inductance will change with frequency, why in the formula inductance below does not include f parameter?

16. ## Re: Calculate inductance of a coil in CST

Any empirical formula involves a simplification. As long as Q is high, frequency dependency of inductance is low and can be usually ignored.

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17. ## Re: Calculate inductance of a coil in CST

Originally Posted by liana82
If the inductance will change with frequency, why in the formula inductance below does not include f parameter?
You need to understand ideal inductors vs. real physical inductors.

Your textbook equation calculates the DC (!) inductance value. At frequencies above DC, there are many influences that change the effective inductance: skin effect will cause a droop (!) in inductance, parasitic capacitances will result in an increase of effective inductance, peaking at SRF where effective inductance goes to infinity.

One comment on FvM: It really depends if the inductor is operating much below SRF. I have been working on EM analysis of PCB inductors and RFIC inductors for many years, and there have been only a few cases like RFID or wireless power transfer where inductance can be assumed constant because operating frequency is much below SRF.

Good luck anyway
Volker

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