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. >>>>>>> >>>>>>>------------------------------------------------------------------= > > ------------------------------------------------------------------ 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