[SI-LIST] Re: Is a copper plane not tied to any net a reference plane?
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <tom@xxxxxxxxxxxxx>, "steve weir" <weirsp@xxxxxxxxxx>
- Date: Wed, 5 May 2004 13:41:47 -0700
GREAT example - thanks! I agree with the prediction for that case (a
"T" of coax). Now I'm trying to relate this to a via in a PCB.
The reason I'm particularly interested in this is that I often do
exactly what Scott described, building test boards with vias to
interface with stripline traces (or traces on the bottom of the board).
While I have been warned that this phenomenon (the resonance) is to be
guarded against, I have yet to see it manifest itself (that I know of -
I must admit I don't spend a lot of time looking for small resonances in
the MHz region). I am at this point trying to decided if I have been
incredibly lucky (and that's generally NOT been my experience) or
there's some other factor(s) that need to be considered before deciding
whether the "resonance" is likely to manifest itself (and I need to take
the appropriate steps to guard against it). =20
I'd love to hear how a person would practically reproduce the effect
(from a via between 2 planes), and the reverse (how to keep it from
happening). Actually, I've heard ALL KINDS of things I should do to
keep it from happening. To date, I've not implemented any of them in my
test boards since I haven't been able to reproduce the phenomenon
myself. I am respectfully skeptical that I'm likely to see it. I think
there are are other factors that aren't being articulated that increase
or decrease the chance of it occuring. Perhaps these might be:
1) the statement "The only difference between the coax and the board is
the impedance"? That seems like a huge difference to me.
2) the phenomenon can only occur on a test board without any vias
connecting the 2 planes (at least to the distances we've been talking
about - 12", for instance). My test boards typically have only ground
planes (they're test boards, after all), and have vias connecting the
planes wherever I have launch structures, and sometimes elsewhere.
Others?
Jeff Loyer
-----Original Message-----
From: Tom Dagostino [mailto:tom@xxxxxxxxxxxxx]=20
Sent: Wednesday, May 05, 2004 1:18 PM
To: Loyer, Jeff; 'steve weir'
Cc: si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Is a copper plane not tied to any net a
reference plane?
This is a very easy experiment to run to see for yourself. Take you
favorite VNA, calibrate it and place a T between the two probes. At the
junction, attach a 2nsec length of coax and sweep S21. You will see a
series of resonances. Now if you double the length of the coax you will
see resonances but at half the frequency of the first. The only
difference between the coax and the board is the impedance. The coax
will be 50 Ohms and the board will be whatever the trace impedance is.
=20
Tom Dagostino
Teraspeed Consulting Group LLC
Device Modeling Division
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx <mailto:tom@xxxxxxxxxxxxx> =20
=20
=20
Teraspeed Consulting Group LLC
2926 SE Yamhill St. =20
Portland, OR 97214 =20
http://www.teraspeed.com <http://www.teraspeed.com> =20
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Teraspeed (SM) is the service mark of Teraspeed Consulting Group LLC
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Loyer, Jeff
Sent: Wednesday, May 05, 2004 12:54 PM
To: steve weir
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Is a copper plane not tied to any net a reference
plane?
So, you would predict that:=3D20
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]=3D20
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=3D20
about 1/500MHz, lambda/2 at 250MHz, and lambda/4 at about 125MHz. A Tr
of=3D20
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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