[SI-LIST] Re: Is a copper plane not tied to any net a referenceplane?
- From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- To: jeff.loyer@xxxxxxxxx
- Date: Wed, 05 May 2004 13:08:54 -0700
Jeff,
No, but if you transition through a plane that has no stitching or
bypass with a via, you will surely see the resonance on the plane, some
of which may be detected on the signal. A plane can have a resonant
mode that is uninteresting until excited. When excited, it may radiate
like the dickens.
scott
Loyer, Jeff wrote:
>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
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>__________________________________
>>>>Do you Yahoo!?
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>
>
>
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--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Teraspeed is the registered service mark of
Teraspeed Consulting Group LLC
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