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Re: [10GBASE-CX4] Comments on initial draft of 10GBASE-CX4 PMD




Mike,

This is not quite "compliance channel", but does it address your
issues?


+-----------+             +----------------+
| TX PMD    | v(t)        |Standard range  |
| device    +------------>| of test load   |
+-----------+    \        +----------------+
                  \
                   \    +--------+     +-------------+
                    --->| G(s)   +---->| time-domain |
                        +--------+     |  template   |
                                       +-------------+

The principles:
(1) TX output v(t) is being constrained by the spec. This is
     defined at the physical connector (interop point)
(2) The constraint on v(t) is expressed as a temporal template
     applied *after* a linear filter
(3) As a design spec, the constraint should be met over a
     full range of legal test loads and legal test signals
(4) As a conformance test, perhaps one test signal (the
     low freq pulse already proposed) and two limit conditions
     on test load are satisfactory.

G(s) is a complex frequency response function which we need to make
up in committee. It need not exactly model a feasible (or nominal)
channel. In fact, if G(s) is optimized for the purpose of ensuring
interoperability without overspecifying, it may deviate quite a
bit from real channel characteristics. 

In any case, let's make sure that we recognize a design spec and
a conformance test are ideally based on the same model but are
*two different things*.

Cheers,
  Chuck Harrison
  Far Field Associates, LLC
  +1 360 863 8340 (voice)  PST = GMT-0800


Mike Jenkins wrote:
> 
> Howard and all,
> 
> First of all, my apologies for jumping in somewhat late on this
> discussion.  I appreciate all the work that has gone into
> 10GBASE-CX4 to date, and I hope my comments will be received as they
> are intended -- as constructive.
> 
> I will list several specific comments below, but my overall
> not-so-hidden agenda is to persuade the group not to abandon the
> XAUI compliance interconnect concept and to avoid required TX
> near-end measurements.  I offer three reasons for  this:
> 
>   1. (The practical reason)  Near-end TX waveforms have maximum high
>      frequency content, exciting any and all fixture resonances.
>      Hence near-end measurements -- especially with restrictive
>      templates -- can be a nightmare of small excursions outside the
>      template.  Yet these high frequency aberrations are attenuated
>      by the transmission path and affect the far-end signal quality
>      very little if at all.  Far end measurements are much more
>      benign and to the point.
>   2. (The theoretical reason)  A jitter budget, with jitter
>      increasing down the transmission path made sense for
>      trapezoidal  waveforms, but not for pre-emphasized TX
>      waveforms.  With pre-emphasis, the jitter can be better at the
>      far end of the path than at the TX output.  Specifying such TX
>      waveforms to guarantee an adequate RX waveform is complex and
>      sensitive to assumptions.
>   3. (The time-to-market reason)  It appears inevitable that the RX
>      will require additional equalization compared with XAUI.
>      Hence, keeping the TX specifications as close as possible to
>      XAUI(at least in format) would seem to offer the lowest risk,
>      fastest route to market for 10GBASE-CX4.
> 
> For what it's worth, a compliance channel approach to TX specs would
> also render the issue of minimum TX amplitude irrelevant.
> 
> Here are some specific comments on sections of the draft:
> 
>    * 54.7.3.2  Load   For accurate measurements of 3.125 Gb/s
>      signals, especially at the TX, 2.5 GHz is probably not adequate
>      bandwidth.  (At the far end, after 12 or 20 dB of attenuation
>      at these frequencies, this is much less of an issue.)
>    * 54.7.3.4 Output Impedance and 54.7.4.5 Input Impedance   I
>      realize these sections are out of XAUI, but I would like to
>      point out that the values specified (if my math is correct) are
>      equivalent to 1.04 pF of dif'l load capacitance on the TX and
>      0.424 pF of dif'l load capacitance on the RX.  These would be
>      problematic limits.
>    * 54.7.3.6 Differential Output Template  (I gather there has
>      already been discussion on this item, but I'll add my two
>      cents.)  This template is unworkably tight.  The definition of
>      normalized amplitude guarantees that the waveform will never
>      exceed +/-1, so template values outside that range are
>      meaningless.  This leaves a mere 7 percent of peak-peak
>      amplitude -- 3.5% at each extreme -- as a target.  With
>      resonances and +/-5% load tolerances, this won't work.  The
>      implied risetimes are also unrealistic.  The slowest risetime
>      of a trapezoidal signal that would fit within the template
>      would have a risetime of 101.4 ps.  Realistic, curved waveforms
>      would need to be even faster.  Of course, all these problems go
>      away with the compliance interconnect approach.
>    * 54.7.3.7 Transmitter Jitter   The added requirement on the mean
>      of jitter distributions will probably invalidate a number of
>      existing jitter measurement approaches.  A lot of capital
>      equipment might be obsoleted with this additional requirement.
>    * 54.8.2 Cable Assembly Insertion Loss  Just some questions
>      here.  I do not understand the relevance of the 1/sqrt(f)
>      term.  This is large at low frequency and smaller at higher
>      frequencies.  Also, it should be stated that "f" in the
>      expression is in units of Hz.  Also, I believe the inequality
>      is in the wrong direction.  And, lastly, the sentence ending
>      "...deviate by more than 10% from equation 54.3." might need
>      some explaining.  54.3 is, after all, an inequality.  Does this
>      allow +/-10% deviation from the right hand side expression?  Or
>      only -10%?
> 
> Again, apologies for the last minute comments.
> 
> Regards,
> Mike
> 
> --
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>  Mike Jenkins               Phone: 408.433.7901            _____
>  LSI Logic Corp, ms/AH260     Fax: 408.433.2840        LSI|LOGIC| (R)
>  1873 Barber Lane          mailto:Jenkins@LSIL.com        |     |
>  Milpitas, CA  95035         http://www.lsilogic.com      |_____|
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
>