Chris yes c) is the running bet for terrestrial links. Accurate assessment of Rj is tricky stuff. Our experts Ransom and Al spend quite a bit of time on it. Best Regards, Steve. At 01:16 PM 7/5/2006, Chris Cheng wrote: >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 ? > >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 > >as seen by the PLL to change then the PLL will move the VCO, thus > >creating a limit to the max observed RJ. If the RJ is distributed > >so that the frequency does not have to change then the 'single > >incremental interval' effect will apply. > > Have we not then got a jitter distribution that is Gaussian in > > 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 ? >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 >different and worth discussing. > >The basis of our disagreement appears to be in the definition of >bound. I look at things from the standpoint of electrical >noise. Time interval in a timing circuit is the result of the >magnitude of some electrical quantity, and is always causal, each >event defining a new interval follows the previous. This means that >noise effectively multiplies the interval by some factor 1/oo <= K ><= oo. Jitter is still unbounded, but every incremental interval has >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 >the VCO then the PLL feedback loop applies gain to divide the effect >of the noise. If we still believe in infinity, infinity divided by >anything is still infinity. In practice will the oscillator stop for >an unlimited time? It will only when it fails. On the other end, >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 >fixed amount of phase to the jittered stream for the case of the >oscillator event, and will take a different, much longer amount of >time to align to the short term frequency offset that noise in the >PLL error amplifier causes. The phase error between the source >stream and the recovered clock in the latter case generally follows a >classic 2nd order step response. The golden PLL is a PLL with >specific frequency response and damping. Even if we have a PLL that >uses N=1, the PLL only starts correcting after a timing error is >already apparent. For a timing error of sufficient magnitude data >moves outside the timing window, a data recovery error is guaranteed, >and no PLL is going to prevent that. A nasty little problem that >gets into systems is power supply noise coupled into the VCO and/or >error amplifier by one means or another. For systems with high Ns it >can get really ugly. > >On a slightly different tack, for a PLL using a PFD, the unit >interval is that at the phase comparator input which is VCO/N or >Fref. Noise whacking the error amplifier will push the VCO off >frequency until new information arrives to get it back. If the noise >jumps the VCO up it can take up to VCO/N cycles before we start >correcting. If noise slows the VCO down, it will take at least one >cycle of Fref to get it back. > >So, I think the only place that we are having any semantic trouble is >on the notion of unbounded noise. While we likely will never see >such a thing, the math really does tell us that an interval can go >virtually to zero 1/oo, or last forever. I think the important part >of this concept is that it says that random noise ( jitter ) will >create data errors sooner or later. And I think doubt about that is >where the discussion began. The tough issue is finding the actual >random jitter. The value is often way overestimated because >deterministic jittter that we have difficulty correlating gets >incorrectly classified as RJ. People turn the crank on the math and >conclude that their links are 10E-12 or 10E-14 when they are really >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 > > an attack on your reply to him. Your definition follows that of FC > > and other serial standards. FC defines random jitter in FC-PI-3 as > >>jitter, random (RJ): Jitter that is characterized by a Gaussian > >>distribution. Random jitter is > >>defined to be the peak-to-peak value for a BER of 10-12, taken to > >>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 > >>jitter of more than -(1UI-epsilon). > >If that jitter is all Gaussian then hasn't it been truncated, or do > >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 > >>distant fixed timing reference, then a stream can theoretically > >>precess total time interval error by an unbounded amount. > >FC measures jitter against a timing reference derived from a golden > >PLL. If over any finite period of time the RJ causes the frequency > >as seen by the PLL to change then the PLL will move the VCO, thus > >creating a limit to the max observed RJ. If the RJ is distributed > >so that the frequency does not have to change then the 'single > >incremental interval' effect will apply. > > Have we not then got a jitter distribution that is Gaussian in > > 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 > >>same lines here. The operation of the oscillator, no matter what > >>its construction is causal. So the closest that any two events can > >>occur is epsilon. That means that any single incremental interval > >>can never have jitter of more than -(1UI-epsilon). > >> > >>If on the other hand we want to integrate phase compared to some > >>distant fixed timing reference, then a stream can theoretically > >>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 > >>> 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 > >>>can measure an infinite time between my reference and a following > >>>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 > >>>occurred an infinite amount of time in the future, but from the > >>>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 > >>>>incremental delay between two events. Imagine for a moment that > >>>>we have a simple relaxation oscillator as the basis of our > >>>>VCO. In the presence of an infinitely large noise pulse, which > >>>>is the limit for random noise, it takes an infinite amount of > >>>>time for the ramp to reach the threshold. The next cycle will > >>>>not begin untilt he current cycle completes. It may sound like > >>>>something from Douglas Adams, but it really is mathematically and > >>>>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 > >>>>>believe you have to take the real world into consideration. > >>>>>Allowing the RJ to be really unbounded means that each edge in a > >>>>>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 > >>>>>reduce until it is zero, it cannot go negative. The time > >>>>>between edges can of course go to +ve infinity, but that isn't a > >>>>>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 > >>>>>>>within the realm > >>>>>>>of times/rates that matter for shipping product? I suppose if > >>>>>>>I sat in my > >>>>>>>chair for long enough, a truly unbounded system might cause a > >>>>>>>gold bar to > >>>>>>>pop into existence on my desk, but my empirical GBR (gold-bar rate) is > >>>>>>>currently 0. > >>>>>>> > >>>>>>>------------------------------------------------------------------ > > > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu