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

  • From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
  • To: "steve weir" <weirsp@xxxxxxxxxx>
  • Date: Wed, 5 May 2004 12:39:45 -0700

If I understand you correctly, you predict that if I ran a trace on a
12" board and performed VNA on that trace, I should expect to see a
"resonance" at 100MHz.  If I took that same trace and ran it on a 24"
board, I would see a resonance at 200MHz, and so on.

If this is the prediction, I am respectfully skeptical.

If this isn't the prediction, could you articulate how the phenomenon
would be reproduced, and how it would manifest itself?

Perhaps you're talking about power distribution (where my experience is
admittedly limited)?  In that case, could you answer the same questions?

Thanks,
Jeff Loyer

-----Original Message-----
From: steve weir [mailto:weirsp@xxxxxxxxxx]=20
Sent: Wednesday, May 05, 2004 11:09 AM
To: Loyer, Jeff
Cc: si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Is a copper plane not tied to any net a
reference plane?

Jeff, sure it is just the propagation velocity of the wavefront.   A 12"

board at Er of 4 is 2nS 1 way delay.  The round trip directly aligns on
a=20
2nS Tr/Tf.

Regards,


Steve.
At 07:25 AM 5/5/2004 -0700, Loyer, Jeff wrote:
>Steve,
>Could you clarify what you meant about "the half-wave resonance of a
12"
>board"?  Do you mean to say that you think a 12" board (I assume you're
>talking about a height or width dimension) will be susceptible to
>resonances at ~100MHz?  If so, I'd like to understand that better.
>
>
>Jeff Loyer
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of steve weir
>Sent: Tuesday, May 04, 2004 3:59 PM
>To: ericsilist@xxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Is a copper plane not tied to any net a
reference
>plane?
>
>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
> >
> >
> >
> >
> >
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