[SI-LIST] Re: Is a copper plane not tied to any net a reference plane?
- From: steve weir <weirsp@xxxxxxxxxx>
- To: ericsilist@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 04 May 2004 15:59:21 -0700
Eric, I believe Dr. Green provided a nice explanation for what happens to
the current in your trace. However, that is hardly the end of the
story. When you reference a signal, intentionally, or unintentionally
against anything other than ground, you would be wise to consider what
happens to that other reference. For your example case, it is pretty close
to the half-wave resonance of a 12" board, resulting in efficient
broadcasts to Zontar and other points East. Resonant cavities excited with
substantial energy are generally not a good thing.
Steve.
At 02:16 PM 5/4/2004 -0700, eric steimle wrote:
>I'm trying to settle an argument without saying which
>side I'm 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
>
>
>
>
>
>__________________________________
>Do you Yahoo!?
>Win a $20,000 Career Makeover at Yahoo! HotJobs
>http://hotjobs.sweepstakes.yahoo.com/careermakeover
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
