[SI-LIST] Re: Do you really ship products at BER 10e-xx ?
- From: "Alfred P. Neves" <al.neves@xxxxxxxxxxx>
- To: <andyp@xxxxxxxxxxxxxxxxxxxxxx>, <Bradley.S.Henson@xxxxxxxxxx>
- Date: Tue, 12 Apr 2005 18:07:11 -0700
Andy,Chris
Approaching this as an ex-applications for a capitol equipment company
that markets jitter solutions, and now working on numerous jitter
related issues with Teraspeed Consulting...
Any real system, with a real transmitter generating some very small
finite amount of Random Jitter, RJ, cannot operate "error free". It is
an issue of probabilities. By definition, RJ is unbounded, therefore
there is always some probability of a failure.
My experience is that I have encountered systems that have occasional
BER problems, and the problems are more than not due to reference
clocks, jitter multiplication due to a string of clocks and excessive
frequency multiplication, and poor signal integrity of an oscillator
(again REFCLK).
The jitter issue, specifically regarding serial links such as
backplanes, can be broken down into 4 primary categories:
1. RJ generated from the transmitter - due to Transmitters VCO and
Reference clock jitter transfer
2. Deterministic jitter (DJ) due to Transmitter - Duty cycle distortion
and Intersymbol interference, periodic jitter due to power supply and
plane resonance
3. DJ due to the physical link - losses in the system (resonance, skin,
dielectric), impedance mismatches, crosstalk, resonances
4. Tolerance of the Receivers - BER measured with combinations of RJ,
DJ and swept PJ (T11.2 Annex A)
Total Jitter is a convolved (added) combination of 1,2, and 3.
Teraspeed Consulting has generated a block diagram that separates the
total jitter into classifications as fine as a single or paired via. A
pair of vias for a differential can be used to relate resonance with
periodic jitter, losses and impedance discontinuities with measurable
DJ.
Of course, if a good transmitter and a well designed link and a receiver
with significant tolerance is incorporated into the design, the actual
BER will appear to be perfect, and it may be directly impractical to
measure. In this case, it may be necessary to add jitter to see how the
system tolerates it with respect to the receivers tolerance. A system
with low RJ and significant DJ, with steep bathtub curves will not start
to have a moderate 1E-8 BER type of problem, it will probably have
catastrophic loss of lock and BER problems. Chris, Andy I think this is
the behavior you were describing, no?
Interestingly, we have measured a significant contribution of jitter
just due to SMA launches in 10Gbpsec systems, compared to well designed
launches such as the one Teraspeed Consulting has designed. The
difference would be dramatic for testing and correlation activity, would
a really poor launch stop a link from working, let's say at 12.5Gbpsec
?????????????????
I would pose an interesting question for Chris - if his particular
system has 1ps RMS more jitter on the REFCLK for a 3.125Gbpsec
transmitter (if it had 1psec RMS initially, it now has 1.414psec RMS
now), would it still meet BER performance for the full link? What is
your confidence it still works? How much BER testing would be required?
How well is his oscillator vendors testing their product for jitter and
phase noise?
How about 30mV more peak-peak switching noise at 400kHz - how tolerant
are the PLL's from losing lock, multiply the higher freq components and
creating a serios PJ problem, how would this impact the Receiver
tolerance - would the system still work, would you now have occasional
failure?
This is not meant to be critical in any way, but unfortunately most BSEE
programs do not require a single class in Stochastic Processes (after
all who in their right mind would elect that class), and that is why a
lot of the engineering community graples with abstract jitter issues.
We have not been trained to think "stochastically".
Alfred P. Neves
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
Hillsboro Office
735 SE 16th Ave.
Hillsboro, OR, 97123
(503) 679 2429 Voice
(503) 210 7727 Fax
Main office
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed is the registered service mark
of Teraspeed Consulting Group LLC
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Andy Pedler
Sent: Tuesday, April 12, 2005 3:36 PM
To: Bradley.S.Henson@xxxxxxxxxx
Cc: Chris.Cheng@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Do you really ship products at BER 10e-xx ?
This is actually right-on topic with a design problem that I'm
investigating. Here's what I require, and maybe someone can suggest
something.
I need a relatively high-speed serial link; let's say 1 Gbps, but if I
can run 2.5 Gbps it will save me cost in another part of the design.
I'd like to run over a backplane, but the design may simply be
board-to-board connectors. It could also be 1-2 foot cables (perhaps
Infiniband type cables). It's a theoretical exercise at this point.
But I can certainly live with 1 Gbps. I can add forward error
correction into my data that is traversing this link, so I can live with
an occasional *single* bit error that comes along once in a blue moon.
But my system will crash and burn if the receiver ever gets a continuous
stream of errors. So I would be happy with a predictable BER of even
1E-7 or 1E-9, so long as the errors are single bit and correctable. But
even 1E-20 is bad if the errors show up in huge numbers all at once.
When I've talked to serdes vendors about how they define BER, I've been
told that these serial links typically operate error free, but every so
often for whatever reason (Chris's cosmic ray), a PLL might get just out
of sync and have to re-lock, and when that happens you get a ton of
errors all at once. Obviously, that will kill my system.
I've built chassis systems with 1 Gbps backplanes and run them for weeks
at a time without recording any errors. But that still doesn't make me
extremely confident that I would *never* see a problem. This system
would have to run for months at a time, and a hiccup would cause a lot
of problems.
Any thoughts?
Andy Pedler - Greenfield Networks
Henson, Bradley S wrote:
> This could make an interesting topic. I have to say that in general, I
> have noticed the same trend: Links work so well the BER is hard to
> determine (lots of test time or link-stress)-or- the links are totally
> messed up. However, I did get called in to troubleshoot a Fibre
> channel application that was just marginal on some of the links. By
> that I mean they would almost make the spec 1E-12 BER sometimes, but
> usually fell short. Some days they operated considerably poorer than
> 1E-12, but not pure garbage.=20
>
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