Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

My Last Answer

This is pointless. 

I do want to comment on A-

The bigger numbers mentioned previously (>30 dB below 16 MHz) are obtained
with very ideal terminations. The small number (15 dB) is obtained with
lousy NEXT from the RJ-45 connectors.

The point (which you seem to be missing) is that the 15 dB isolation makes
what you do on the 4-5, 7-8 pairs much less critical.

And of course it has to be looked at.

I have answered "B- " at least 4 or 5 times, that is enough. It is not a
shifting goalpost, the tests are on the IEEE Web for all to see.

Do what you think best.


At 11:33 AM 5/15/00 -0700, R karam wrote:
>Hi Larry
>I take it the measurements you have done that show 15db isolation
>using the network analyzer, terminated 4,5 and 7,8 with 100 ohms,
>differential and the proper common mode impedance to ground?
>I trust your measurement at this point, but as we  hang the DTE
>on there, Are we not terminating with a different impedance than
>100ohms, and what does that do to the measurement?, I don't
>expect a lot of drama but worth a look...
>How  does "Noise, Inluding RF and xyz..." injection on the used pairs get
>because I have a dc power on there?, and whatever happened to the awsome
>40db common mode rejection of the magnetic and the differential input stage
>of the receiver?  This is a twisted pair after all isn't? all right I get
real unlucky
>and my smart "noise" source injects differential noise now, I am in trouble 
>with or without current saved by how good of a phy I have chosen what Am I
>Larry, I ate drunk and slept phys ask your favorite vendors.
>at the end of the day, PHYs are HUMAN, they are not perfect, and will
never be.
>the hope is that if the phy has 30-40m of margin built in above 100m physical
>length that all this worries still apply but is not as dramatic.
>I hear you, the "rules" are you need a proof, how many Phys and how many revs
>must I go through, and in what system?  I have seen great phys act way better
>in some boxes, it is all in the eye of the "Beholder"....
>As for the rules, I once played checkers with this person, and as I
started to win
>here and there  he kept changing the rules, I tell you that was more fun
than winning
>at the end of the day...   
>my 2c,
>At 10:50 AM 5/15/00 -0700, Larry Miller wrote:
>>Roger and all,
>>What I am saying is that whatever ill effects (not just noise, but rf-type
>>things such as parasitic phenomena such as resonances, impedance matching,
>>and the like) of attaching power components to the signal pairs are
>>intrinsically isolated from the signaling pairs by using the other pairs
>>for power. 
>>This is a basic consequence of the guaranteed properties of CAT-5 cable.
>>The amount of isolation guaranteed as a minimum is per the numbers quoted
>>in my little drama.
>>Even with things like RJ-45 connectors in the link, the isolation that is
>>obtained is at least 15 dB. You can confirm this by putting some CAT-5
>>cable on a network analyzer. (Maybe we will have to do that for the
>>What this means is that whatever dirties of ANY kind your power attachment
>>does to the wires you use (4-5,7-8) is isolated from the signal wires
>>(1-2,3-6) by at least 15 dB over the 2-100 MHz frequency range. This is a
>>factor of more than 5:1 and represents a risk reduction of that amount.
>>15 dB covers quite a lot of sins!
>>In the early meetings of the DTE Committee this was discussed and deemed to
>>be a worthwile thing.
>>I am not saying that it is impossible to successfully attach power to the
>>signal wires, OK?
>>But what I am saying is that if you want to take the riskier path and throw
>>away this 15 dB cushion, you will have to show that you did not mess up the
>>system, at least if you want my vote.
>>***** Slight topic shift *****
>>The present UTP universe is defined by the clauses in IEEE802.3 and ANSI
>>TP-PMD that I cited in the list I put on the IEEE Web site. This is a list
>>of tests that we use to evaluate any new technology variant. 
>>In particular, we (our company) have insisted that PHY vendors go through
>>this hoop. A subset of this same list of tests is what the UNH IOL test
>>suites uses for qualifying new PHY designs.
>>The reason for doing this is that it is possible (even probable) for a link
>>to still operate even though it did not meet these criteria; i.e., a plain
>>old functional test or even general scope pictures do not give any idea of
>>the margins (if any) present. If you have not been doing these tests on
>>your own designs and have been getting away with it, then you can thank
>>others who did do these characterizations so that by simply buying and
>>hooking up "approved" parts you got a compliant design.
>>Now we are going into another dark corner of the PMD ("new technology"), so
>>we have to go back through the whole list of parameters to make sure that
>>we did not inadvertently tank something.
>>There is a definite proof that this is so: some of the test methods sitting
>>in full force and effect in IEEE 802.3 will not work with powered signal
>>wires, and if this is adopted the existing standards will have to be
>>overhauled in some areas.
>>As an analogy, this is a little bit like the Gigabit fiber committee
>>(802.3z) discovering far, far into the standards process that the paucity
>>of modes in laser outputs caused very severe effects on some kinds of
>>multi-mode fiber. This was nothing new, and in fact it was in my
>>20-year-old optics handbooks, with pictures even. But it had gotten
>>overlooked. The result was that it delayed the standard by some 6 months to
>>a year.
>>So, to repeat:
>>You wanna power the signal pairs? Then you gotta show that this does not
>>goof up anything of importance, and in fact, the standard will have to
>>enumerate what you have to do to achieve this.
>>At 09:50 AM 5/15/00 -0700, R karam wrote:
>>> Hi Larry
>>>Please clarify the statement below.
>>>>I hope that it was shown that sending power over the signal pairs raises
>>>>the technical difficulty bar by the amount of isolation between pairs. (15
>>>>dB at least)
>>>are you saying that because power is on the used signal pairs, I get
>>>possibly up to 15db of noise injected on there, what about the supply
>>>filter that insures DC- Pure DC on the wire, I am not clear on this?