Re: [Fwd: Xaui jitter tolerance]
(To FC folks - this message is essentially the same message I sent to T11.2 earlier today).
Mike - refer to Annex G of the FC-MJS report for an explanation for 1667. It is something I wrote, so it should be questioned...
A lot of the history relates to SONET. I personally would be very reluctant to take a radical departure until someone really understands why SONET did what they did and what the tradeoffs were/are.
The 100ppm requirement would be less of an issue with higher corner frequency, not more.
Your point about over-tracking (your CJTPAT discussion) is valid. If the CDR is too fast and tracks every little bit of noise and jitter that comes along, we're asking for trouble. If we think about CDRs as high-pass filters, complex jitter with fundamentals below the corner frequency gets "differentiated", which can effectively double the peak to peak jitter the CDR has to dissipate. (Think about sending a low frequency square wave through a high-pass RC filter). There has been a lot of work done on this, both analytically and in the lab.
> Subject: Re: Xaui jitter tolerance
> Date: Wed, 29 Nov 2000 20:09:02 -0800
> From: Mike Jenkins <jenkins@xxxxxxxx>
> Organization: LSI Logic
> To: stds-802-3-hssg@xxxxxxxx
> References: <200011292314.RAA00398@xxxxxxxxxxxxxxxxxxxxxxxx>
> Richard, Ed,
> I disagree with the proposal to increase the jitter corner frequency
> higher than baudrate/1667. First, the number 1667 is tied to the
> assumed +/-100 ppm reference frequency tolerance. I can't say it's
> an inviolable law of physics, but 1667 does make sense. (I'll try
> to dig up the explanation, if you want it.)
> Second, tracking jitter isn't always a good idea. One Fibre Channel
> jitter test pattern, CJTPAT, is designed to shift edges late, then
> early, repetitively. If the RX follows, it's jitter tolerance is
> reduced compared to averaging out this input jitter and keeping the
> sampling position fixed.
> If you would like more feedback on your proposal, I would suggest also
> posting it to the Fibre Channel phy layer reflector, t11_2@xxxxxxxxxxxx
> Ed Grivna wrote:
> > Hi Richard,
> > I agree with your viewpoint. Many/most RX PLLs for this speed have
> > BW in the 10+ MHz range, which is what you want to allow fast pull-in
> > and tracking of jitter in high noise environments. The low BW is
> > generally a carry over from SONET environments where the
> > repeater functions (and the jitter gain in some parts of the PLL transfer
> > function) can cause problems. As far qas I know, these implementations
> > are all re-timed to a local reference so jitter gain is not an issue.
> > Regards,
> > Ed Grivna
> > Cypress Semiconductor
> > > Since XAUI jitter will likely be addressed in a separate meeting at Austin,
> > > I would like to raise the issue of modifying the jitter tolerance frequency
> > > "break point" from the standard baudrate/1667 (used in MJS) to something
> > > significantly higher.
> > >
> > > For Xaui, the baudrate/1667 would give us a tolerance break point at 1.875
> > > MHz. My feeling is that there is nothing magical about the baudrate/1667
> > > and that it doesn't accurately reflect typical receiver operation in today's
> > > monolithic PLL's. (Perhaps in early telecom days SAW filter applications
> > > required this, but today's receiver designs (at least in XAUI) will not be
> > > using such costly techniques.) Moving the jitter tolerance break point out
> > > to ~5 MHz or so would allow us to track more of the jitter components and
> > > perhaps even make the Tx design easier (smaller capacitors, etc.).
> > >
> > > Soo, would there be any objections to moving the tolerance break point out?
> > > I'd like to get some feedback on this before the Austin meeting if
> > > possible.
> > >
> > > - Richard Dugan
> Mike Jenkins Phone: 408.433.7901 _____
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