I'm in digest mode, so maybe there has been more recent discussion,
but it seems to me you are over-complicating this situation.

They've told you to run a 5mil trace over a ground trace that is
5mils over a ground trace,
and to keep that pair away from everything else.

That is either a microstrip or an imbedded microstrip,
(which your Polar calculator DOES have)
depending on whether you are running the impedance controlled line
on a surface layer or inside. I would consider putting the ground
on the outside with the trace underneath for added protection.

The only considerations I can see are:
1) My calculator result is closer to 60 ohms, you may want to verify
the numbers
2) The ground trace should be wider. We used 3x the trace width at
a previous job, something about the field strength and the square
of the distance.

good luck,
hope I'm not helping some guy in India working for $3 an hour
who took this job away from me

(a former Intel designer)
Jack


> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of John Parsons
> Sent: Wednesday, November 19, 2003 11:14 AM
> To: [log in to unmask]
> Subject: [TN] Co-planar Impedance Calculations
>
>
> Greetings all,
>
> I have a customer who is doing an impedance design for Intel and he has
> no previous impedance experience.  This is the information I have.
>
> - double sided board
> - target impedance 50ohm
> - signal line 5mil wide, spaced 5mil from "ground" trace and this pair
> of traces should be spaced a minimum of 20mil from adjacent traces.
>
> This is what he was given from Intel.  I am using an older Polar model
> (CITS25 calculator) which does have models for co-planar designs but
> from what I can tell they assume that the signal trace is sandwiched
> between ground (return lines) on both sides.  Is this a correct
> interpretation?  Is it possible to model the design as described above?
>
> While we have some experience with controlled impedance I have no
> experience with the aforementioned design and could use some assistance.
>
> Best Regards
> John Parsons

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