## [SI-LIST] Re: Is a copper plane not tied to any net a referenceplane?

• From: Lei Luo <lluo3@xxxxxxxxxxxxxx>
• To: eric steimle <ericsilist@xxxxxxxxx>
• Date: Tue, 4 May 2004 17:33:53 -0400 (EDT)

```Eric,

If you have microstrip TLs on SIG1, you can measure Z0 of this TL by
TDR. Without copper layer on top, it's a microstrip line. With it, it's
a strip line. You will find different Z0 for these 2 cases.

I have similar experiences. The copper layer above it(even not tied to
any references) will change the Z0 a lot.

Hope this helps,

Lei
ECE - NCSU

On Tue, 4 May
2004, eric steimle
wrote:

> I=92m trying to settle an argument without saying which
> side I=92m on, and I was hoping someone could give a
> quick and easy example to prove this.
>
> You have a four layer board that looked like this
>
> SIG1
> GND
> VCC
> SIG2
>
> And the maximum rise time on the board was about 2ns
> (say a 100MHz clock), then if I ran a trace from one
> IC to another on SIG1 the return current would flow
> along the GND plane (assuming no splits in the plane
> etc.)
>
> What if I covered SIG1 with a plane  of copper say
> hovering 5 mil above SIG1, and then made the distance
> between SIG1 and GND 100mil so..
>
> Plane sheet of Copper
> 5 mil of air
> SIG1
> 100 mil FR4
> GND
> VCC
> SIG2
>
> Now that plane sheet is not GND and it has no
> association to any net it is just a continuous sheet
> of copper about the size of the board.  One of us
> argues that the return current will continue to flow
> along the GND plane as it did before.  The other
> argues that the return current will instead flow
> mostly along that sheet of copper unitl it jumps back
> to the GND plane as it gets closer to the IC.  Any
> help in this would be much appreciated.
>
> Thanks
>
>
>
>
>
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Lei Luo
Electrial & Computer Department
NC State University

Room438 EGRC, Centennial Campus,
Office: 1.919.513.2015  Mobile: 1.919.749.9262

Welcome to my webpage: http://www4.ncsu.edu/~lluo3
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```
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