RE: [SI-LIST] : Coplanar Transmission Line

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From: sam.kvk@amd.com
Date: Thu Feb 03 2000 - 18:24:34 PST


Hi Ron,

Let me see if I can help.

The electric field in a dielectric material doesn't change its polarity that
easily as it does in air. Dielectric materials require some additional
energy to change the field polarity that are associated with alternating
currents. Such energy is derived from the travelling wavefront and
dissipated as heat (resistive loss) inside the dielectric material. Thus, it
is obvious that such energy loss keeps increasing with increasing frequency
of the signal.

Dielectric resistive loss is usually specified by the "loss tangent" for
that material and is a function of the dielectric constant "sigma".

Hope the RF gurus agree with me.

Best Regards,
Sam KVK
AMD, Singapore

> -----Original Message-----
> From: Ron Miller [SMTP:rmiller@Brocade.COM]
> Sent: Thursday, February 03, 2000 6:02 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Coplanar Transmission Line
>
> 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: owner-si-list@silab.eng.sun.com
> > [ <mailto:owner-si-list@silab.eng.sun.com>]On Behalf Of WAUGH,RAY
> > (HP-SanJose,ex1)
> > Sent: Thursday, February 03, 2000 3:34 PM
> > To: 'si-list@silab.eng.sun.com'
> > 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: ray_waugh@agilent.com
> >
> > USPS: Agilent Technologies
> > Wireless Semiconductor Division
> > 39201 Cherry Street, MS NK20
> > Newark, California 94560
> > ------------------------------------------------------
> >
> >
> > -----Original Message-----
> > From: Zabinski, Patrick J. [ <mailto:zabinski.patrick@mayo.edu>]
> > Sent: Thursday, February 03, 2000 12:49 PM
> > To: si-list@silab.eng.sun.com
> > 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
> ==========0000-(_)0000===========
> Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA
> 95131
> rmiller@brocade.com, rbmiller@sjm.infi.net
>

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