Re: [SI-LIST] : Coplanar Transmission Line

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From: Roy Leventhal (
Date: Fri Feb 04 2000 - 07:11:41 PST

SI Gurus,

Many thanks to all who have responded to my request for in formation on CPW. All
of the responses indicate great skill and knowledge.

As in so much of what these subjects address, optimization of CPW over MSL/SL
depends on implementation. There would appear to be a learning curve to come up.
Both in the design (facilitated by ACCURATE simulation) and manufacturing
(facilitated by experience).

If I understand the responses thus far, it would appear that CPW is inherently
better when pushing clock speeds beyond 500 MHz until, it too, runs out of gas
(where?). But, this is only true if you know what you're doing. Meaning you
would have to design and optimize an interconnect configuration in some detail.
Meaning simulation with today's schedules.

Using the aid of a simulator to design CPW is important precisely because the
respondents thus far have indicated dielectric and copper losses are very
dependent on the specific geometry and materials of the implementation. I would
expect the modeling of dispersion, skin effect and the manner in which the EM
field is solved would all be important for simulation accuracy. 3-D solution
still being too slow, a practical simulator would probably still be 2-D. I would
guess that the 2-D EM field would have to be solved in the plane of the
transmission line as opposed to solved normal to it to be accurate for CPW.
These are just a couple of givens that come to my mind.

It would appear that traditional RF/MW simulator companies would be the ones who
have addressed these givens. Plus, there are the historical givens of the types
of problems they have been addressing. Also, the recent discussions of receiver
jitter, ground bounce and EMI issues would indicate the need to turn to such
simulator companies for companies engaged in high speed digital work.

Unfortunately, these RF/MW companies don't yet seem to speak "signal integrity
(time domain, first switch, etc.)" really well as yet. This shouldn't be
surprising. I'm fortunate to know what S11 is when they proudly explain how they
solve for it and plot it. But, there is quite a communications gap to bridge if
I'm trying to communicate with the majority of digital engineers. Communications
gaps always cause problems.

I'm back to a long-term learning curve to bring the tools, techniques,
experiences, suppliers, etc., in-house here to bring in CPW and apply it in a
high speed digital culture.

Again, many thanks to all,


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