RE: [SI-LIST] : CPW and CPS

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From: Jian X. Zheng (jian@zeland.com)
Date: Mon Jan 22 2001 - 08:47:21 PST


Hi, Bob:

I agree with you that there is some bizarre mode problems in CPW. However,
as a differential transmission line, there is one thing CPW will be much
better than a CPS: It is the balancing in the differential transmission
line. Using CPS, when you have a bend, the even mode will be excited and it
is hard to remove it especially when the ground plane is present. Some
designers try to use another bend to compensate the length difference. It
helps reducing the synchronization problem. However, you just can not remove
it on a CPS (with ground plane). However, on CPW, it is easy to balance the
differential line by using bond wires at the proximity of the
discontinuities:

Microstrip is simple. However, its high frequency performance is not as good
as stripline, CPS and CPW because of its high dispersion at high frequency.

Stripline is also simple and its transmission line properties are very good.
However, discontinuities such as vias in a stripline circuit may cause
severe radiation.

The point is that each type of transmission lines may have its advantage and
disadvantage.

Modern electromagnetic simulation tools can yield very accurate results and
design on microstrip, stripline, CPS, CPW. You can check out our full-wave
IE3D electromagnetic simulation tool from our web site. It is excellent for
different simulation and design of different types of circuits. Thanks!

Best regards,

-----------------------------------------------------------------------
Jian-X. Zheng, Ph.D
Zeland Software, Inc., 48890 Milmont Drive, 105D, Fremont, CA 94538, U.S.A.
Tel: 510-623-7162, Fax: 510-623-7135, 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 Bob Lewandowski
> Sent: Friday, January 19, 2001 6:58 PM
> To: Chris Padilla
> Cc: Scott McMorrow; SI-LIST@silab.eng.sun.com
> Subject: Re: [SI-LIST] : CPW and CPS
>
>
> CPW can have some bizarre mode problems that are not intuitive.
> Particularly,
> large multilayer structures. What's ground, and what's not is
> frequently the
> issue. Also getting a clean, mode free, launch is a bigger
> problem then with
> microstrip or stripline. Dimensional tolerances are much tighter
> also. We did
> it with thin film on ceramic.
>
> The old software that I used along time ago (equation based) left
> a lot to be
> desired. Cut and try was the order of the day. New 3D solvers
> should help a
> lot.
>
> ---Bob Lewandowski
> Vixel Corp.
>
> Chris Padilla wrote:
>
> > Scott,
> >
> > In my young career as an EMI engineer, I have never seen anyone
> use CPW or
> > CPS. These waveguides seem mostly reserved for and used by the
> microwave
> > community. Is there any good common literature out there on the web
> > somewhere regarding these seemingly little used waveguides? We
> have tons
> > and tons of stuff on microstrips and striplines (and the slight
> variations
> > of those) but I don't see much on the CPW or CPS. CPS =
> coplanar striplines
> >
> > ----->Chris
> >
> > >Aubrey,
> > >*<snip>*
> > >Another solution which works quite well is to use CPW or
> > >grounded CPW construction for diff pairs on outer layers.
> > >
> > >
> > >regards,
> > >
> > >scott
> >
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