Re: CRPAT / CJPAT Pattern Question
All,
I want to step back a second to point out what this conversation
is aimed at. Changing the pattern on three lanes to see the effect
at the receiver of the 4th lane is manipulating far-end crosstalk
(FEXT). FEXT is much smaller than near-end crosstalk (NEXT) because
the inductive and capacitive components subtract in FEXT but add
in NEXT. The NEXT for a receiver is created by the transmitters
in the same device as the receiver under test. If these transmitters
are sending CRPAT (or whatever) asynchronously, all possible
combinations will occur to close the receiver eye. Extraordinary
efforts to manipulate FEXT are probably for relatively rather small
returns in additional eye closure.
Regards,
Mike
pat_thaler@xxxxxxxxxxx wrote:
>
> Michael,
>
> >From my experience testing crosstalk, single frequency stimulus signals
> would not be an effective way of testing crosstalk. Generally, the received
> crosstalk from a disturber such as an adjacent signal path is the sum of
> many crosstalk components each coupling in with its own phase. Because of
> this, the crosstalk is not a smooth function with frequency. It bounces
> around staying under an envelope where the envelope is the amplitude you get
> when all the components add in phase. Therefore, testing crosstalk at a few
> discrete frequencies doesn't tell you much about where the envelope lies.
>
> Generally one wants to test crosstalk with a signal containing a broad
> spectrum of frequencies (e.g. CRPAT or even the normal idle signal since we
> designed that to spread energy across the spectrum). One of the reasons put
> forth in favor of having individual lane disables was that one could test
> crosstalk from one lane by disabling the other transmitters. Therefore, we
> don't need to do any special patterns for crosstalk measurement.
>
> Pat
>
> -----Original Message-----
> From: Michael Debie [mailto:mdebie@xxxxxxxxxxxxx]
> Sent: Thursday, June 14, 2001 9:47 AM
> To: 'DAmbrosia, John F'; Michael Debie; 'Serial PMD reflector (E-mail)'
> Subject: RE: CRPAT / CJPAT Pattern Question
>
> John,
>
> Absolutely agree. I was just simplifying my description to one lane, but, I
> assumed we would test each lane individually. As far as the pattern
> selection is concerned, the use of different patterns on each lane allows us
> to see the contribution each of the other lanes has on cross talk noise.
> For example, suppose Lane 1 was driving a /5 clock like pattern (1111100000)
> the FFT of the jitter on the Lane under test would show a spectral line at
> Fc/10 and the amplitude of the spectral line would be the pk-pk impact on DJ
> that Lane 1 has on the Lane Under Test (LUT?). We could set up the other 3
> lanes with varying degrees of clock like patterns and quickly estimate each
> lanes contribution to crosstalk on the LUT. We could perform this test on
> all 4 lanes to measure crosstalk contribution. It would also be interesting
> to sweep through several clock like frequencies on the non tested lanes to
> quantify the impact of crosstalk as a function of instantaneous frequency.
> The test in which we apply the same pattern on all of the non tested lanes
> will tell us how the crosstalk components combine.
>
> Regards,
> Michael
> -----Original Message-----
> From: DAmbrosia, John F [mailto:john.dambrosia@xxxxxxxxxxxxxxxxxxx]
> Sent: Thursday, June 14, 2001 9:11 AM
> To: 'Michael Debie'; 'Serial PMD reflector (E-mail)'
> Subject: RE: CRPAT / CJPAT Pattern Question
>
> Michael,
> I think your second proposal makes more sense, but i think it would need to
> go one step further. I think we should cycle which lane is the "different"
> lane like this-
>
> Pat 1A Pat 1B Pat 1C Pat 1D
> Lane A + - - -
> Lane B - + - -
> Lane C - - + -
> Lane D - - - +
>
> Where the "+" lane would be the pattern, and the "-" would be the
> compliment. Thus, all channels get examined. If only 1 lane is tested,
> then the test is specific to the implemenation, where if all lanes in a
> channel get examined, then the performance of the channel is fully examined
> rather than 1/4 of it.
>
> John
>
> -----Original Message-----
> From: Michael Debie [mailto:mdebie@xxxxxxxxxxxxx]
> Sent: Wednesday, June 13, 2001 5:41 PM
> To: DAmbrosia, John F
> Subject: RE: CRPAT / CJPAT Pattern Question
>
> John,
>
> A good diagnostic for cross talk would be to place different frequency clock
> like patterns on all of the lanes. This could tell us the amplitude of
> cross talk per other lane and where it comes from. Also, if we run the same
> patterns on three lanes and one lane different, we could see how the other
> three lanes combine to effect cross talk on the lane under test.
>
> Regards,
> m
>
> -----Original Message-----
> From: DAmbrosia, John F [mailto:john.dambrosia@xxxxxxxxxxxxxxxxxxx]
> Sent: Wednesday, June 13, 2001 4:21 PM
> To: Serial PMD reflector (E-mail)
> Subject: CRPAT / CJPAT Pattern Question
>
> Everyone,
> The 10GEA XAUI Interoperability Group met this week, and were discussing the
> use of the CRPAT / CJPAT patterns for its testing. A general observation
> was that the same data pattern appear on all 4 lanes synchronously, which
> means crosstalk is not really being testing, which was probably being
> accounted for by connector crosstalk budget of 4%. Tyco presented data
> http://grouper.ieee.org/groups/802/3/ae/public/may01/dambrosia_2_0501.pdf
> that showed that crosstalk, which resulted from signals switching in-phase
> (i.e. high to low or low to high), could actually improve the overall
> response of the system. Thus, the resultant eye is improved and would be
> best case, and not even nominal (all adjacent channels quiet).
>
> Obviously, there are a lot of system variables that come into account when
> considering crosstalk, but it would seem that we could improve the harshness
> of these patterns by not making all 4 channels have the same data patterns.
>
> John D'Ambrosia
> Manager, Semiconductor Relations
> Tyco Electronics Corporation
>
> Tel. 717.986.5692
> Mobile 717.979.9679
>
> email - john.dambrosia@xxxxxxxxxxxxxxxxxxx
>
>
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