Re: [SI-LIST] : Coplanar Transmission Line

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From: Ron Miller (rmiller@Brocade.COM)
Date: Thu Feb 03 2000 - 17:51:53 PST


Jian

If you will check MDS or ADS you will find HP's definition by their models
to be coplanar waveguide with or without a ground plane. No mention of
coplanar strips. Since they stole their models and terminology from the
origional research in the MTT Journals, I would tend to side with them.

Ron Miller

Jian Zheng wrote:

> Hi,
>
> I noticed that some people in the SI list consider two coupled strips
> (without ground plane) as co-planar waveguide. However, in the microwave
> terminology, two coupled strips without ground plane is called co-planar
> strips (or CPS). Three strips with two gaps are normally called co-planar
> waveguide (CPW).
>
> Compared to normal microstrip structures (strip over a substrate over a
> ground plane), both CPS or CPW may have less dielectirc loss. The reason is
> that about 50% of the field will be in the air and dielectric loss is less
> important. For microstrip, about 60-95% of the field will be in the
> substrate and the dielectric loss is higher. As metallic loss (loss in the
> metal), CPW normally has less loss too because its metallic path is normally
> wider. It is hard to say for CPS because it really depends upon how wide the
> strip is.
>
> Some clear advantage of CPW and CPS over microstrip are:
>
> (1). CPW and CPS have less dispersion than microstrip. In the other word,
> their equivalent circuit parameters (LRCG) are more constant over a wide
> frequency range.
>
> (2). Another advantage is that it is easier to achieve short circuit which
> are common in microwave: On microstrip, you have to build the via holes for
> the short circuit. On CPW and CPS, you can build them on the surfaces.
>
> One important issue of CPW is the balance of the mode(s). Theoretically,
> there is one fundamental mode (cut off frequency at DC) for a two-conductor
> transmission line system. There are multiple fundamental modes for a
> 3-conductor system. For microstrip and CPS, there are two conductors. Only
> 1-fundamental mode exists. As long as the connectors are far away from
> discontinuities (microwave terminology for interconnects of different
> cross-section shape in the transmission line), only one single balanced
> differential mode can exist. For CPW, there are 3 conductors. There are
> multiple fundamental modes: (1) -1 1 -1; (2) -1 0 1; (3) -1 -1 1....
> Normally, people use the 1st fundamental mode. However, any discontinuities
> such as a bend can induce the other fundamental modes. People normally use
> wire bonds to connect the -1 conductors to suppress the other higher order
> modes.
>
> I am not sure where you can find text books for this topic. Thanks!
>
> Best regards,
> ----------------------------------------------------------------------------
> ------------
> Jian-X. Zheng, Ph.D
> Zeland Software, Inc., 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 Roy Leventhal
> > Sent: Thursday, February 03, 2000 10:58 AM
> > To: si-list@silab.eng.sun.com
> > Subject: [SI-LIST] : Coplanar Transmission Line
> >
> >
> >
> >
> > Dear SI Experts,
> >
> > Who out there has worked with coplanar (waveguide-like)
> > transmission lines as
> > opposed to stripline/microstripline?
> >
> > What can you tell me about it other than what I can find in
> > Collins or Bahl &
> > Bhartia?
> >
> >
> > Thanks in advance,
> >
> >
> > Roy
> >
> >
> >
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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
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rmiller@brocade.com,  rbmiller@sjm.infi.net

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