Re: [SI-LIST] : Coplanar Transmission Line

From: Ron Miller ([email protected])
Date: Thu Feb 03 2000 - 18:01:33 PST

Hi Jian

It appears that we differ again. My understanding of the dielectric losses is
that the losses are magnetic/eddy current rather than electrostatic capacitive in nature.
It is produced by the nature of the material and has nothing to do with capacitance.

Perhaps a materials literate person is on the list and can fill in the details. Help??

ron miller

Jian Zheng wrote

> 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,
>
> ----------------------------------------------------------------------------
> ------------
> 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
> ----------------------------------------------------------------------------
> ------------
>
> > -----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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Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
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