[SI-LIST] One stitching via or more vias is better for 25Gbps

  • From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 8 Sep 2014 09:39:50 -0600

Guys-
 

When a diff signal transitions from one Vss return plan to another, the
return currents of the diff signal mostly overlap in the cavity and a pure
diff signal does a poor job of exciting cavity resonances between the two
Vss planes.

 

It is not necessary to have "stitching" vias in a cavity if you are only
transitioning differential signals.

 

However, it is impossible to engineer a real interconnect that is so
symmetrical as to not have any mode conversion. This means there will always
be some common signal components along with the diff signals. It's the
common signals which will excite the cavity modes, as Steve has been
emphasizing.

 

When you do a simulation, unless you are clever enough to anticipate a
possible asymmetry in the interconnect and include it in the simulation, you
will not generate common signals in your simulation and the diff response
through the cavity may look just fine. 

 

How big an issue is the common signal in the cavity? .it depends. If you are
really, really lucky, you may have only -30 dB of common signal (3% of your
diff signal) and the common noise injected in the cavity modes may not be
enough to couple anywhere and screw up the system. 

 

But, if you are not so lucky, you may have -15 dB common signal. This is
still within many specs, not a problem by itself, but will contribute 15% of
the diff signal as common current into the cavity and screw up channel to
channel cross talk and ground bounce to some other channel or couple out as
EMI.

 

If you want a robust system, use shorting vias in the cavity to suppress
modal resonances- how many? It's not just about 1 or 2 per diff via. The
shorting vias are to suppress model resonances. Their spacing is important.
You will see the first model resonance with a wavelength equivalent to 6 x
the via to via spacing. For a 1 GHz lowest excited mode, this is 1 inch
apart.

 

If you want to rely on luck in your system, maybe the first few, maybe even
the first 20 boards will work, but as soon as you have too much mode
conversion in a channel, your product may fail. 

 

If you are interested in this topic, I cover it in the Advanced Gigabit
Channel Design (AGCD) class in the SI Academy, (www.beTheSignal.com) and in
my graduate course at CU. I plan to post my lectures from this course on the
SI Academy by the end of the year.

 

--eric

 

 

 

 

*******************************************************
Dr. Eric Bogatin, Dean

 <http://www.bethesignal.com/> Teledyne LeCroy Signal Integrity Academy

 <http://www.bethesignal.com/> www.beTheSignal.com 

Adjunct Prof, ECEE Univ of Colorado, Boulder

105 S Sunset St, Suite J

Longmont, CO 80501 USA

Twitter @beTheSignal
e:  <mailto:eric@xxxxxxxxxxxxxxx> eric@xxxxxxxxxxxxxxx

cell: 913-424-4333 
******************************************************

Msg: #6 in digest

From: David Banas <DBanas@xxxxxxxxxx>

Subject: [SI-LIST] Re: One stitching via or more vias is better for 25Gbps

Date: Mon, 8 Sep 2014 14:38:15 +0000

 

On 9/7/2014 8:49 PM, leeritchey@xxxxxxxxxxxxx
<mailto:leeritchey@xxxxxxxxxxxxx%3cmailto:leeritchey@xxxxxxxxxxxxx>
<mailto:leeritchey@xxxxxxxxxxxxx> wrote:

> All of the things proposed don't seem to have any basis.  I, on the

 

> other hand, have built test boards to find out.  My findings are

 

> available in my classes and do not show such vias have any value.

 

> 

 

Hi Lee,

 

Our simulation results suggest that, while these stitching vias have no
discernable effect on signal integrity at the receiver at these speeds, they
do noticeably affect electromagnetic emissions from the PCB edges,
potentially threatening electromagnetic compliance testing failure, when
they are omitted. And I was wondering if you have any experimental findings,
in this regard, that you can share.

 

Thanks!

 

David Banas

Sr. Member of the Technical Staff

Altera Corp.<http://www.altera.com/>

 

 

 



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