Coplanar Waveguide - RF Cafe Forums

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cubeleg
Post subject: coplanar waveguide Posted: Tue Dec 05, 2006 2:11 pm

Lieutenant

Joined: Tue Dec 05, 2006 1:55 pm
Posts: 3
Hi!

We are trying to calculate the characteristic impedance and the speed of light in a coplanar waveguide (CPW), which has been loaded placing a material in the top of the CPW. I have several questions about how to calculate this parameter with a network analyzer, the most important are the following:

i. will this system be well described using the telegraphist equations?

ii. how can I extract the scattering matrix of the system CPW-loaded CPW-CPW without the effect of connectors? We have a calibration kit and we are able to get rid of the wires which are carrying the signal to the CPW, but not the connectors.

If someone can give me a reference where I can find the basics of CPW would be great.
Thanks!
Cubeleg

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IR
Post subject: Posted: Tue Dec 05, 2006 3:41 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany

Quote:
. will this system be well described using the telegraphist equations?

The telegraphist equations define any transmission line, doesn't matter of which kind.

Quote:
ii. how can I extract the scattering matrix of the system CPW-loaded CPW-CPW without the effect of connectors? We have a calibration kit and we are able to get rid of the wires which are carrying the signal to the CPW, but not the connectors.

I would use a known reference plane like a known length transmission line, measure its properties, i.e. full S-parameters. Then add the connector and measure again the same. This should give you in detail the delta caused by the addition of the connector.

Do these tests at the same frequency in which you measure the CPW structure.

This site contains a lot of useful information and applets:

http://www.amanogawa.com/transmission.html

_________________
Best regards,

- IR

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cubeleg
Post subject: Posted: Wed Dec 06, 2006 8:14 pm

Lieutenant

Joined: Tue Dec 05, 2006 1:55 pm
Posts: 3
Thank you for the answers and the link, they clarify what we are trying to do.

But I have to complete my first question, because the telegraphist equations state the behavior of a transmission line. I understand that the material placed in the top will change the Z and the electrical lenght of the loaded line, due to the change in the effective permittivity-permability, then the scattering of the CPW are different, but, will the step in the CPW, or other geometric factor, modify the scattering parameters at the interface CPW-loaded CPW?
I ask this because we are trying to recover the Z and electrical lenght of the loaded line, in order to characterize the electromagnetic parameters of a material, but applying the same equations that we use to recover this factors in a coaxial line filled with different materials, we have no success. I would like to clarify if equations are the same or I have to worry about more factors.

Cubeleg.

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IR
Post subject: Posted: Thu Dec 07, 2006 3:58 am

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Why not trying to use a EM solver to find out these parameters?

The telegrpahist equations are valid for the same conditions, namely for the same substrate, because they are derived from the substrate parameters: L, R, C, G are the substrate's parameters per given length. These are the entry parameters for the equations. If you load your CPW with a different material then the equations for the different material are different and the total result is different.

If the step is made from the same substrate as the CPW then you should use the same parameters in the equations as you used for the CPW.

_________________
Best regards,

- IR

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cubeleg
Post subject: Posted: Thu Dec 07, 2006 5:07 am

Lieutenant

Joined: Tue Dec 05, 2006 1:55 pm
Posts: 3
Thank you very much, now is clear what to do, I'll use what you suggest.
Regards
Cubeleg

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