[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 12:54:25 -0700

So, you would predict that:=20
If I ran a trace on a 12" board and performed VNA on that trace, I
should expect to see a "resonance" at 250MHz.  If I took that same trace
and ran it on a 24" board, I would see a resonance at 125MHz, and so on.
(note that this time I have F indirectly propotional with board size -
an oversight in my last posting).

If this is the prediction, I am respectfully skeptical.

And by the way, I didn't mean to imply that my experience in any other
field besides power delivery was UN-limited!  That's only true for the
field of inducing errors in measurements and simulations.

Jeff Loyer

-----Original Message-----
From: steve weir [mailto:weirsp@xxxxxxxxxx]=20
Sent: Wednesday, May 05, 2004 12:45 PM
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, no, and I don't know where you get that idea.  Lambda will be at=20
about 1/500MHz, lambda/2 at 250MHz, and lambda/4 at about 125MHz.  A Tr
of=20
2ns has a knee at about 250MHz, which coincides with lambda/2.

Regards,


Steve.
At 12:39 PM 5/5/2004 -0700, Loyer, Jeff wrote:
>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]
>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
>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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