RE: [802.3ae] XAUI jitter tolerance test
Would almost agree with the first statement, except that the noise that is generated by the multipliers would be bounded and therefore according to our definitions deterministic. Could be they are still coloured in the spectrum, but they are not Gaussian.
Definately agree with the second statement, the DJ has a very complicated spectrum which at least theoritically can go to very low frequency, such that the CDR starts tracking low frequency components from "killer patterns". This is exactly the reason why one shouldn't have a CDR BW that is too high, otherwise you will start tracking wander that is not actually there. (i.e. it's down converted HF jitter).
From: Lindsay, Tom [mailto:tlindsay@xxxxxxxxxxxxxxxxxxxx]
Sent: Freitag, 17. Mai 2002 22:07
To: Qicheng Yu; stds-802-3-hssg@xxxxxxxx
Subject: RE: [802.3ae] XAUI jitter tolerance test
I'll try this.
1. The purpose of the statement was to help ensure that someone would not create RJ with mostly low frequency content that would be tracked out by the tested CDR. So, in that sense, the spec is intending to allow high frequency RJ and to ensure that a CDR can appropriately dissipate it.
In reality, RJ will probably not have a lot of high frequency content, and in fact may have frequency content dominated below a few MHz. Sources include clock synthesis, multipliers, etc. The spec assumed that if a CDR can tolerate high frequency RJ, it can also tolerate low frequency RJ.
2. I am still putting some thought into this, but I believe it is too simple to say that deterministic jitter is mostly high frequency. I agree that common mechanisms that create DJ may be high frequency (such as high frequency rolloff in the channel, DCD, etc.), but the jitter spectrum generally tracks the spectrum of the data itself. Indeed, CJPAT creates significant low frequency jitter when passed through a lossy channel.
Also, mechanisms such as inadequate low frequency response also map into low frequency DJ. But again, the jitter spectrum will tend to match that of the data.
(Of course, a benefit of 8B10B is that its spectral content rolls off at the low end when compared to scrambled codes).
So, in real systems, you will have to tolerate other jitter sources - both RJ and DJ.
(425) 672-8035 x105
From: Qicheng Yu [mailto:qiyu@xxxxxxxxxx]
Sent: Friday, May 17, 2002 5:05 AM
Subject: [802.3ae] XAUI jitter tolerance test
I have a question on the jitter tolerance test signal for XAUI receiver. In P802.3ae/D4.3, section 18.104.22.168 Jitter Tolerance, on page 305, it is said "Random jitter is calibrated using a high pass filter with a low frequency corner of 20MHz and 20dB/decade roll off below this." Does this mean the random jitter component of the jitter tolerance test signal is mostly high frequency, above 20MHz? If so, since deterministic jitter is also by nature high frequency, the amount of jitter a receiver has to tolerate at, say, 2MHz, would be mostly the 0.1UIpp sinusoidal jitter and not much more. Is this correct? Thanks for your attention.