[SI-LIST] Re: risetime effects of plane breaks
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: istvan.novak@xxxxxxx, silist <si-list@xxxxxxxxxxxxx>
- Date: Fri, 21 Jan 2005 11:00:55 -0500
Istvan
Yes, you are absolutely correct. That's actually what I see in
simulations of a split island. A slot mode exists to propagate
aggressor crosstalk onto a victim, which can be viewed as a noise
waveform which travels in the transverse direction on the planes along
the slot, and a refleted waveform which propagates in the backwards path
of the signals. Since, in my simulation, the slot is continuous and
completely surrounds a power island, there is a resonant mode which also
causes crosstalk peaking. This can be considered as noise on Supply A.
Then, the same slot phenomena occurs on power island. Lets call it
Supply B. In its case, since it is a closed structure a different
resonant mode exists, and thus a different frequency domain noise
profile. In effect, noise from Supply A exists on the victim trace
simultaneously with noise from Supply B. And yes, the results is
extremly complex, sometimes being additive and sometimes subtractive
depending on the phasing of the original sources and the shape and size
of each resonant structure.
One can take the attitude that all noise on power/ground cavities is a
result of some sort of split plane phenomenal, or what I have called in
the past "power mode conversion." For example, a trace crossing a plane
is a well-known split plane mode conversion phenomena. A power or
signal via penetrating a plane cavity can be viewed as a split plane
crossing in three dimensions. A connector or package with poorly
designed high speed return paths can also be looked at as having the
same characteristics ... and so forth.
As my good friend and consultant Jeff Walden points out, split planes
and RF coupled edges are hideous creatures, since they have the added
property of being efficient radiators.
regards,
scott
Istvan Novak wrote:
>Scott,
>
>One more possible consideration is the extra plane noise introduced in
>the signal loop.
>
>Whenever we cross a split in a power plane, and have to assume that the
>two power
>domains have uncorrelated rail noise, in worst case we add the noises
>introduced
>by the two planes. We know we should not reference a very noisy plane,
>and it is also
>clear that best is to use the signaling's own supply rail as a second
>reference beyond
>ground. But having split power planes we end up with a second power
>rail probably
>not related to our signal. Unless this second rail is much quiter than
>the signaling's own
>supply rail, the worst-case pp noise will be the some of the two noise
>contributions.
>
>Regards,
>Istvan
>
>
>Scott McMorrow wrote:
>
>
>
>>When this thread started I was on vacation. However, I found this
>>interesting enough to resurrect some previous simulations I'd performed
>>in CST Microwave Studio. After much playing, twiddling and generally
>>having fun I can say several things:
>>1) It's pretty easy to confirm Doug's results using 3D fullwave
>>simulation. In fact, in about 30 minutes I can replicate his case and
>>create a design that can be easily modified for many other
>>possibilites. The microstrip split plane crossing is a no-brainer.
>>Just don't do it and expect anything approaching an EMI "clean" system.
>>
>>2) Chris and Steve ... and eventually myself, wanted to know more about
>>the various different stripline plane crossing configurations, so I
>>setup a simulation with a VDD island not unlike what might be found in a
>>memory system, and performed multiple simulations with dual asymmeteric
>>stripline crossing the plane twice on it's way to the memory module. Not
>>surprisingly the following is true:
>>
>> It is best not to cross a split plane ... even with stripline.
>> If you do, it is better to cross a split that is adjacent to a
>> ground plane
>> It is even better if you cross a split adjacent to a ground plane on
>> the stripline layer furthest away from the split plane (i.e. next to
>> a ground plane)
>> It is worst to cross a split plane that has no adjacent ground.
>> The width of the gap in the plane makes very little difference until
>> it becomes really small or really big.
>> Crosstalk scales almost linearly with the number of aggressors
>> crossing the split. (i.e. - it can get really bad!)
>> Bypass of the split power island helps for frequencies below 500
>> MHz, provides no help for frequencies higher than 500 MHz, and as
>> such has no benefit to most of the noise and crosstalk created by
>> high speed signals crossing onto and off of the island.
>>
>>The energy released into the power/ground plane cavities by high speed
>>signal split plane crossings is huge and essentially cannot be
>>suppressed with bypass capacitors. Any attempt at supprerssion with
>>capacitors exhibits what I call a "Whack-A-Mole" property. You can
>>never get rid of those pesky little moles. All you can do is to move
>>them around by thumping them. Given that all this energy is rattling
>>around the PCB power planes from split plane crossings, it will
>>eventually go somewhere. Since it's really easy to develop all sorts of
>>resonant power island cavities that have primary resonant frequencies in
>>the 500 MHz to several GHz range, it is not at all unlikely that any
>>split plane crossing has an extremely strong potential to excite a
>>resonance in a frequency range that will cause most systems to fail EMC
>>compliance testing About all you can do is to shield the cavity patches
>>using ground layers. This should reduce the radiated energy
>>significantly, but will not totally eliminate it, since eventually it
>>will find it's way to all those pesky device and package leads.
>>
>>
>>best regards,
>>
>>Scott
>>
>>
>>
>>
>>
>
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Scott McMorrow
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