[SI-LIST] Re: Is a copper plane not tied to any net a reference plane?
- From: "Lynne Green" <lgreen22@xxxxxxxxxxxxxx>
- To: "'eric steimle'" <ericsilist@xxxxxxxxx>
- Date: Tue, 4 May 2004 15:28:12 -0700
Eric-
Physically, "return current" flows in any and all adjacent metal as
capacitive/displacement current, and coupled inductance current.
Only the DC return current is restricted to flow in a power or ground
plane (and even that has some caveats).
Copper pour that is not tied to anything can also make an excellent
source of crosstalk noise. (I often think of it as a very_wide
ungrounded guard trace.)
- Lynne
Dr. Lynne Green
Green Streak Programs
http://www.greenstreakprograms.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Lei Luo
Sent: Tuesday, May 04, 2004 2:34 PM
To: eric steimle
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Is a copper plane not tied to any net a reference
plane?
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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