# RE: [SI-LIST] : Coplanar Transmission Line

From: Jian Zheng ([email protected])
Date: Thu Feb 03 2000 - 17:19:56 PST

Hi,

From electromagnetics, loss in any dielectrics (substrate or metallic strip)
is calculated as:

Loss = Volume_Integration ( Sigma * E * E ) where Sigma is the conductivity
and E is the electric field.

Loss on a transmission line is mainly determined on the percentage of the E
field in lossy dielectrics. A bitmap picture is attached to this e-mail to
illustrate the cross-section of microstrip and CPW (co-planar waveguide).
For microstrip, most of the field (significantly more than 50% of the field)
is concentrated between the strip and the ground or the substrate. For CPW,
about 50% of the field will be in the air and the rest is in the substrate.
In fact, if the substrate is thin, more than 50% of the field will be in the
air (see the attached picture). There is no doublt about it that the
dielectric loss is lower on CPW.

Interestingly, when frequency increases, more field will be concentrated
into the substrate for microstrip line (an in-direct indication is that the
effective dielectric constant Ereff of the waveguide is increasing with
frequency), it makes microstrip line more frequency dependent. However, the
field pattern in the cross-section of CPW is almost unchanged with
frequency. That is the reason why CPW has less dispersion.

As metallic loss is concerned, the loss is more related to the cross section
area of the middle strip of the CPW and the cross-section area of the
microstrip. If they are identical, the metallic loss should be very close.
This is in fact ohm's law says at DC frequency.

Certainly, if you do not force identical voltage between the two side (or
ground) strips in the CPW, you will create other modes mentioned in my
previous e-mail. The other modes will have different propagation speed and
they will cause dispersion to the signals and it is not what you want.

You may ask why the propagation speed of the other modes are different. It
is also related to the field distribution in the cross-section or the
percentage of the field in different dielectrics. Thanks!

Best regards,

----------------------------------------------------------------------------
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Jian-X. Zheng, Ph.D
Zeland Software, Inc., 39676 Mission Blvd., Fremont, CA 94539, U.S.A.
Tel: 510-797-8109, Fax: 510-797-8241, Web: http://www.zeland.com
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> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of WAUGH,RAY
> (HP-SanJose,ex1)
> Sent: Thursday, February 03, 2000 3:34 PM
> To: '[email protected]'
> Subject: RE: [SI-LIST] : Coplanar Transmission Line
>
>
> I doubt if losses in the metal are significant. In my experience (as a RF
> and microwave circuit designer), losses in dielectric (particularly FR4)
> dominate total losses in symmetrical stripline, microstrip and coplanar
> waveguide. I would not worry much about skin effect when your
> dielectric is
> so lossy.
>
> I have found that CPW losses are generally about the same as
> those found in
> a comparable microstrip. Keeping the potential on both CPW ground strips
> exactly the same can be tricky, however.
>
> Ray
> ------------------------------------------------------
> Raymond W. Waugh - WSD Diode Applications
> E-mail: [email protected]
>
> USPS: Agilent Technologies
> Wireless Semiconductor Division
> 39201 Cherry Street, MS NK20
> Newark, California 94560
> ------------------------------------------------------
>
>
> -----Original Message-----
> From: Zabinski, Patrick J. [mailto:[email protected]]
> Sent: Thursday, February 03, 2000 12:49 PM
> To: [email protected]
> Subject: RE: [SI-LIST] : Coplanar Transmission Line
>
>
> Possibly.
>
> If the CPW line width (and conductivity, roughness, etc.) is
> the same as microstrip/stripline ***AND*** the CPW ground
> traces are adequately wide, then the metal losses are
> the same (within engineering tolerance anyway).
>
> However, some compromise on the ground strip widths is generally
> needed, which will incease the effective inductance and resistance
> of the return path, which in turn increases the effective loss
> of the line. There are rules of thumb to follow in terms of
> how wide the grounds need to be to avoid this, but you won't
> hear them from me.
>
>
> Pat
>
> >
> > Will the skin loss not be higher in CPW than in microstrip/stripline?
> >
> > Vinu
>
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