[SI-LIST] Re: Fibre channel interconnect margins

  • From: "Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>
  • To: <weirsi@xxxxxxxxxx>, "David Instone" <dave.instone@xxxxxxxxxx>
  • Date: Wed, 5 Jul 2006 13:16:44 -0700

Steve,
I start off by asking the same question before, "does your company ship =
products with 10e-12 error rate ?" and it seems it comes down to the =
following answers :

a) "Its a fact of life, I'll try to keep it to 10e-x13,14,15 or better."

Well, I still want to know what that number is ? And when you have a =
customer that has a peta byte of installation, how does that translate =
into actual errors per second in your FCAL ?=20

b) It may happen in some part of the link but overall the link can =
tolerate it

In particular,
>If over any finite period of time the RJ causes the frequency=20
>as seen by the PLL to change then the PLL will move the VCO, thus=20
>creating a limit to the max observed RJ.  If the RJ is distributed=20
>so that the frequency does not have to change then the 'single=20
>incremental interval' effect will apply.
>  Have we not then got a jitter distribution that is Gaussian in=20
> form but with limits to the maximum deviations?

I don't know about FCAL jitter tolerance spec but let's take the OC-xx =
spec as a reference, what happen when the jitter spectrum passes ft ? Is =
the jitter still bounded ?
To take it out to a non-ideal world, how does one trade off jitter =
tolerance (for example, by Rj and Dj) and jitter accumulation (for =
example, by PLL supply noise) ? You only have one choice of loop =
dynamics in your PLL and you can not help one without hurting the other. =
And I would bet jitter accumulation from PLLVDD dwarfs the concern over =
jitter tolerance. Just look at how elaborate those PLLVDD distribute =
schemes we have (isolation traces, filters etc). Can you still say your =
Rj is unconditionally bounded by such PLL's ?

c) 10e-12 happens only in methodology, you are really running a BERT =
much much lower, possibly beyond the life of the system or the component =
or reasonable measurement technique

Which, I think, is essentially what you are saying in the last sentence. =
Do we really claim a 10e-12 simply because we got an erroneous Rj from =
an instrument in a Dj dominated system, multiply the sigma to 14 or 15 =
times per the BERT spec and scare ourselves to such error rate ?=20
And before the instrument guys jump on me again, let me be clear that I =
don't think the instrument is at fault but rather the problem lies on =
the application (Dj dominate system measurement) that force the =
instrument to report back a number that it cranked out based on some =
fixed assumptions not suited for such situation.

For those who claim a), my hats off to you because you are a braver man =
than me to publicly say that.

For those who claim b), I sure would like to know what kind of PLL you =
have

And my money will be on c).

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Tuesday, July 04, 2006 3:40 PM
To: David Instone
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Fibre channel interconnect margins


David, I did not regard it as an attack just an opinion that is=20
different and worth discussing.

The basis of our disagreement appears to be in the definition of=20
bound.  I look at things from the standpoint of electrical=20
noise.  Time interval in a timing circuit is the result of the=20
magnitude of some electrical quantity, and is always causal, each=20
event defining a new interval follows the previous.  This means that=20
noise effectively multiplies the interval by some factor   1/oo <=3D K=20
<=3D oo.  Jitter is still unbounded, but every incremental interval has=20
a positive duration.

So far we have been talking about noise in the oscillator itself.

Now, let's see what a PLL does to this quagmire.  If noise hammers=20
the VCO then the PLL feedback loop applies gain to divide the effect=20
of the noise.  If we still believe in infinity, infinity divided by=20
anything is still infinity.  In practice will the oscillator stop for=20
an unlimited time?  It will only when it fails.  On the other end,=20
two successive pulses can occur essentially on top of each other.

A receiver PLL will take a finite amount of time to realign within a=20
fixed amount of phase to the jittered stream for the case of the=20
oscillator event, and will take a different, much longer amount of=20
time to align to the short term frequency offset that noise in the=20
PLL error amplifier causes.  The phase error between the source=20
stream and the recovered clock in the latter case generally follows a=20
classic 2nd order step response.  The golden PLL is a PLL with=20
specific frequency response and damping.  Even if we have a PLL that=20
uses N=3D1, the PLL only starts correcting after a timing error is=20
already apparent.  For a timing error of sufficient magnitude data=20
moves outside the timing window, a data recovery error is guaranteed,=20
and no PLL is going to prevent that.  A nasty little problem that=20
gets into systems is power supply noise coupled into the VCO and/or=20
error amplifier by one means or another.  For systems with high Ns it=20
can get really ugly.

On a slightly different tack, for a PLL using a PFD, the unit=20
interval is that at the phase comparator input which is VCO/N or=20
Fref.  Noise whacking the error amplifier will push the VCO off=20
frequency until new information arrives to get it back.  If the noise=20
jumps the VCO up it can take up to VCO/N cycles before we start=20
correcting.  If noise slows the VCO down, it will take at least one=20
cycle of Fref to get it back.

So, I think the only place that we are having any semantic trouble is=20
on the notion of unbounded noise.  While we likely will never see=20
such a thing, the math really does tell us that an interval can go=20
virtually to zero 1/oo, or last forever.  I think the important part=20
of this concept is that it says that random noise ( jitter ) will=20
create data errors sooner or later.  And I think doubt about that is=20
where the discussion began.  The tough issue is finding the actual=20
random jitter.  The value is often way overestimated because=20
deterministic jittter that we have difficulty correlating gets=20
incorrectly classified as RJ.  People turn the crank on the math and=20
conclude that their links are 10E-12 or 10E-14 when they are really=20
more like 10E-20 from an RJ standpoint.

Regards,


Steve.
At 06:35 AM 7/4/2006, David Instone wrote:
>Steve,
>  Firstly, my initial response was in support of Alan's posting not=20
> an attack on your reply to him.  Your definition follows that of FC=20
> and other serial standards. FC defines random jitter in FC-PI-3 as
>>jitter, random (RJ): Jitter that is characterized by a Gaussian=20
>>distribution. Random jitter is
>>defined to be the peak-to-peak value for a BER of 10-12, taken to=20
>>be approximately 14 times
>>the standard deviation of the Gaussian distribution.
>
>
>  So lets look at it both ways
>>That means that any single incremental interval can never have=20
>>jitter of more than -(1UI-epsilon).
>If that jitter is all Gaussian then hasn't it been truncated, or do=20
>we have to say that it's not RJ because it's bounded?
>
>
>>If on the other hand we want to integrate phase compared to some=20
>>distant fixed timing reference, then a stream can theoretically=20
>>precess total time interval error by an unbounded amount.
>FC measures jitter against a timing reference derived from a golden=20
>PLL.   If over any finite period of time the RJ causes the frequency=20
>as seen by the PLL to change then the PLL will move the VCO, thus=20
>creating a limit to the max observed RJ.  If the RJ is distributed=20
>so that the frequency does not have to change then the 'single=20
>incremental interval' effect will apply.
>  Have we not then got a jitter distribution that is Gaussian in=20
> form but with limits to the maximum deviations?
>
>Regards
>Dave
>
>steve weir wrote:
>>David,
>>
>>I would just like to make certain that we are talking along the=20
>>same lines here.  The operation of the oscillator, no matter what=20
>>its construction is causal.  So the closest that any two events can=20
>>occur is epsilon.  That means that any single incremental interval=20
>>can never have jitter of more than -(1UI-epsilon).
>>
>>If on the other hand we want to integrate phase compared to some=20
>>distant fixed timing reference, then a stream can theoretically=20
>>precess total time interval error by an unbounded amount.
>>
>>Regards,
>>
>>
>>Steve.
>>At 03:10 AM 7/4/2006, David Instone wrote:
>>>Steve,
>>>   I didn't disallow an infinite time between events.  I allow for=20
>>> the time between events to be between 0 and infinity, but not =
negative.
>>>Thus if I'm measuring the time between edges and my reference I=20
>>>can measure an infinite time between my reference and a following=20
>>>edge but never more than 1 UI between the last edge and my reference.
>>>That last edge could of course be from a edge that should have=20
>>>occurred an infinite amount of time in the future, but from the=20
>>>point of view of the measurement it's only 1 UI early.
>>>Regards
>>>Dave
>>>
>>>
>>>steve weir wrote:
>>>>David, I disagree.  It does not change causality.  It changes the=20
>>>>incremental delay between two events.  Imagine for a moment that=20
>>>>we have a simple relaxation oscillator as the basis of our=20
>>>>VCO.  In the presence of an infinitely large noise pulse, which=20
>>>>is the limit for random noise, it takes an infinite amount of=20
>>>>time for the ramp to reach the threshold.  The next cycle will=20
>>>>not begin untilt he current cycle completes.  It may sound like=20
>>>>something from Douglas Adams, but it really is mathematically and=20
>>>>physically sound.
>>>>
>>>>Regards,
>>>>
>>>>Steve.
>>>>At 01:50 AM 7/4/2006, David Instone wrote:
>>>>>Because it makes for a nice simple clean definition.  However, I=20
>>>>>believe you have to take the real world into consideration.
>>>>>Allowing the RJ to be really unbounded means that each edge in a=20
>>>>>bit stream could be advanced or delayed by an infinite amount.
>>>>>Taken to extremes this means that  the order of  edges  could be =
reversed.
>>>>>This is obviously absurd, the measured time between edges can=20
>>>>>reduce until it is zero, it cannot go negative.  The time=20
>>>>>between edges can of course go to +ve infinity, but that isn't a=20
>>>>>bit error, the system has failed or been switched off.
>>>>>steve weir wrote:
>>>>>>RJ really is unbounded by definition.
>>>>>>
>>>>>>Steve.
>>>>>>At 09:46 AM 7/3/2006, Steven Kan wrote:
>>>>>>
>>>>>>>>Date: Fri, 30 Jun 2006 21:48:56 -0700
>>>>>>>>From: Alan.Hiltonnickel@xxxxxxx
>>>>>>>>Subject: [SI-LIST] Re: Fibre channel interconnect margins
>>>>>>>>
>>>>>>>>In fact, I think that companies DO ship products that toss a =
random
>>>>>>>>error approximately every 10e-xx or so. Why? Because the =
statistical
>>>>>>>>theory behind those errors is that random/Gaussian noise is, by
>>>>>>>>definition, unbounded - errors are a fact of life, even if the =
error
>>>>>>>>rate is very low.
>>>>>>>I suppose we're way off in the weeds, here, but is the noise =
actually
>>>>>>>unbounded? Or does it just behave in a Gaussian-like manner=20
>>>>>>>within the realm
>>>>>>>of times/rates that matter for shipping product? I suppose if=20
>>>>>>>I sat in my
>>>>>>>chair for long enough, a truly unbounded system might cause a=20
>>>>>>>gold bar to
>>>>>>>pop into existence on my desk, but my empirical GBR (gold-bar =
rate) is
>>>>>>>currently 0.
>>>>>>>
>>>>>>>------------------------------------------------------------------=

>
>

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