RE: [SI-LIST] : CPW and CPS

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From: Daniel, Erik S. (Daniel.Erik@mayo.edu)
Date: Fri Jan 19 2001 - 11:35:20 PST


Chris-

We have often designed PC boards with ground metal coplanar with microstrip
and striplines to provide additional inter-trace isolation. We usually keep
the spacing from the coplanar ground to the trace large enough that the
edge-edge coupling is not too large so that the traditional microstrip and
stripline planes provide the majority of the ground return. Therefore, I
suppose one could argue whether these are "true" coplanar waveguide (CPW) or
finite ground coplanar (FGC) structures, but they are nonetheless useful.

I did once make a test board with "true" CPW structures (i.e., a
ground-signal-ground arrangement with no back side ground), but found that
it was somewhat more difficult to get well controlled near-50-ohm impedances
because of the much stronger sensitivity of the impedance of CPW structures
to variations in metal thickness, soldermask thickness and dielectric
constant, etc. -- properties which are not too well controlled in PCB
processes, but which can be controlled much better in thin film process
technologies such as those used to make RF/MMIC devices.

                                        - Erik

==================================================================
Erik Daniel, Ph.D. Voice: (507) 538-5461
Mayo Foundation Fax: (507) 284-9171
200 First Street SW E-mail: daniel.erik@mayo.edu
Rochester, MN 55905 Web: www.mayo.edu/sppdg/
==================================================================

> -----Original Message-----
> From: Chris Padilla [mailto:cpad@cisco.com]
> Sent: Friday, January 19, 2001 12:35 PM
> To: Scott McMorrow
> Cc: SI-LIST@silab.eng.sun.com
> Subject: Re: [SI-LIST] : CPW and CPS
>
>
> 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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