[ibis-macro] Re: Question on seeting the EMD direction

  • From: Mike Steinberger <msteinb@xxxxxxxxxx>
  • To: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • Date: Thu, 26 Jun 2008 18:20:23 -0400

Scott-

The whole area of crosstalk analysis is one in which our industry hasn't had a lot of collective experience yet. There are several possible techniques, and not a lot of agreement yet as to which techniques are appropriate for which problems.

I'll be presenting a paper on the subject at the IEEE EMC conference in Detroit this August. So as to avoid clogging up the e-mail reflector, I'll send you a copy privately, and will be happy to send anyone else a copy if they're interested.

Mike S.

Scott McMorrow wrote:
Mike

So now let's disregard a mechanical change to the system configuration and let's look at crosstalk impact on equalizer tap settings. How do you propose this be modeled and analyzed? How do you guarantee that you have selected the best possible settings?


Scott

Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



Mike Steinberger wrote:
Scott-

As regards mechanical configurations, I believe we agree.

In particular, yes, we agree that if a channel can be put in an unfavorable mechanical configuration, that mechanical configuration must be analyzed.

That still isn't a very good motivation for adding to the complexity of an analysis for the majority of applications because someone could think up an unusual case.

Mike S.

Scott McMorrow wrote:
mike

Where this becomes an issue is in equalizer training with low S/N ratio at the receiver, where very small changes in equalizer settings can result in very large changes in received BER. Some equalization training algorithms assume that the system is LTI during the training period. This may not be true for a card hot-swaped into a fully operational system, where crosstalk from adjacent channels is marginal at best. In order to test an equalization training algorithm pretty much requires that the time varying behavior of the network be considered. I've seen continuous linear analog equalizers that can be "tricked" by crosstalk into unstable operation.

In the case of the vibrating vehicle and a twin-ax cable, it is quite possible to move the cable to a position where the multi-conductor transmission line develops a serious mode conversion issue at just the wrong frequency, causing a nasty S12 null. This happens with cables that have poor mechanical construction, or that are close to the edge of their operational range. If a mechanical vibration causes the shield to move a very small amount, it will drastically change the differential properties. Clearly this would cause increased BER, and it may be possible to re-equalize out of the problem, by boosting high frequency response within a specific frequency range.

These are just considerations that designers must consider. (For example, selection of a cabling system with better mechanical stability.) But, since companies on this committee are tasked with developing modeling and simulation software to handle these sorts of communication channels, and generating equalizer coefficients is one of the most important tasks that has to be accomplished, then it might be necessary to consider non-LTI interconnect.


regards,

Scott


Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



Mike Steinberger wrote:
Arpad-

Taking either Scott's case of a vibrating vehicle or Richard Ward's even higher frequency case of a vibrating disk drive, how does the highest frequency of vibration compare to the data rate of the channel? I'm having a hard time imagining a significant harmonic component above about 100kHz. Maybe I don't have much imagination, so let's make it 1 MHz..

Still, with that many orders of magnitude difference between the mechanical frequencies and the electrical frequencies, any change in electrical wave shape due to mechanical movement is going to be insignificant, so the LTI approximation is still a good one.

What I think these examples do suggest is that the designer of a system that has mechanical vibration must analyze the channel performance for a representative sample of the mechanical configurations that will occur over the course of a cycle of vibration. Thankfully, most of us won't have to execute that procedure..

I'm reminded of an old saying about user interfaces: "If an interface is so easy to use that any fool could use it, only a fool would." I think a similar statement applies to engineering languages: "A language which is capable of expressing any engineering problem is too complex to be useful for any engineering problem."

My 2c.
Mike S.

Muranyi, Arpad wrote:
In our recent discussions on Interconnect-SPICE and our last
IBIS-ATM teleconference the question was raised whether we can
safely assume interconnects are LTI.  Scott's message below
(from a different thread) seems to indicate that there are
situations when this may not be the case.  This makes me
nervous about writing a specification that by its definition
would disallow those effects to be simulated...  Any comments?
Arpad
===============================================================

------------------------------------------------------------------------
*From:* Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
*Sent:* Thursday, June 26, 2008 2:03 PM
*To:* Muranyi, Arpad
*Cc:* twesterh@xxxxxxxxxx; msteinb@xxxxxxxxxx; wkatz@xxxxxxxxxx; huangchunxing@xxxxxxxxxx; IBIS-ATM; guantao@xxxxxxxxxx
*Subject:* Re: [ibis-macro] Re: IBIS-AMI

Arpad

I agree with you, and would add that even the analog network characterization portion of a passive interconnect system is not a well understood topic. Several issues come to mind:

    * Time variance of the interconnect due to:
          o Temperature
          o Humidity
          o Mechanical Vibration (think twin-ax cables in a vehicle)
* Equalizer training in the presence of crosstalk and noise spikes
      in a hot-plug environment

But heck, I'd settle for an IBIS-AMI model that correlates with something that can actually be measured.

Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
Muranyi, Arpad wrote:
Sorry to everyone for arguing a little on what has
been stated so far.  I tend to disagree with the
statement(s) that the analog corner modeling is
a well understood topic.

Just because it is common practice to use high/low
supply voltages with low/high temperatures, etc...
to achieve best/worst timings, it doesn't mean that
this practice actually gives the best/worst timings.
Having done large amounts of parameter sweeps while I
was working for my previous employer, I saw solution
space plots which had failing islands corresponding
to "in-between" parameter values, and not at the
extremes.

The answer then was that we do not know where the
system is failing unless we simulate all possible
combinations of parameter values.  That's when the
frequency domain (resonance) analysis, sensitivity
analysis, design of experiments (DOE) and similar
techniques became popular.  I tend to believe that
the application of statistical analysis techniques
is a natural continuation of this evolution, as
kind of an attempt to reduce the amount of time it
takes to go through all possible parameter combinations...

My $ 0.02 worth...

Arpad
===========================================================

-----Original Message-----
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Todd Westerhoff
Sent: Thursday, June 26, 2008 1:00 PM
To: msteinb@xxxxxxxxxx; scott@xxxxxxxxxxxxx
Cc: wkatz@xxxxxxxxxx; huangchunxing@xxxxxxxxxx; 'IBIS-ATM';
guantao@xxxxxxxxxx
Subject: [ibis-macro] Re: IBIS-AMI

It's worth pointing out an IBIS-AMI model actually has two models -

- the analog model in the .ibs file, and
- the algorithmic model pointed to by the .ibs file

IBIS-AMI analysis separates analysis of a link into two stages - network
characterization and link
analysis.  The analog model is used for the former, while the
algorithmic model is used for the
latter. When we talk about modeling PVT variation, it's worth considering how
PVT affects each type of model
separately. Modeling the effects of PVT on a transmitter's output stage
or a receiver's input stage
is well understood (it's what IBIS has been doing for years), while
modeling the effects of PVT on
equalization behavior is [relatively] new ground.  I agree this is
something we should consider
standardizing, and I also agree that we need more experience in this
area before it makes sense to
try & put such standards in place.

Todd.

Todd Westerhoff
VP, Software Products
SiSoft
6 Clock Tower Place, Suite 250
Maynard, MA 01754
(978) 461-0449 x24
twesterh@xxxxxxxxxx
www.sisoft.com


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