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Re: Xaui jitter tolerance

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

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