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RE: Cross Talk

Hi Roger,
Regarding cross talk issue, to my opinion the situation is more complicated
since there is more than one port caring power
to the media and it is not clear to me what will be the impact of crosstalk
and other noise issues in the following cases:

1.  If a single power supply output is connected to the center tap of the
data transformer (Phantom connection)
     and obviously we have more than one Data+Power output (12 or 24 etc..)
than we can say that all center taps of all 
     data transformers are "shorted"  together at high frequency signals.
     (Each Receive-Transmitt center tap of the transformer is shorted by the
power supply output capacitor. All the Receive
     transformer center taps are shorted together and the same applies for
all the transmit center taps.)

2.  Same as 1, but all outputs has separate power supply output. In this
case there is some parasitic capacitance between each
     DC output although they are isolated from each other.

3.  Add to the above description the fact that from EMI reasons there is
about 1 to 10nF between power supply output to 
     power supply chassis ( = system gnd) thus the construction of the
connections may not be balanced.

Questions raised from the above cases are:

1. What will be the impact on the requirements for cross talks between one
port to other ports, 
    is there a problem here?
2. Is there any restrictions in the 802.3 standards (or any other standard)
for zero voltage isolation between each data path 
   to an  other port data path?
    (If there is,  We will need isolated power supply for each port)

What is your opinion regarding the above issue? 


Darshan Yair
Chief Engineer 
PowerDsine Ltd.  -  The Intelligent Telecom Power
POB 7220, Neve Ne'eman Industrial Zone, Hod Hasharon 42451, Israel
Tel: +972-9-775-5100, Ext. 210,  Fax: +972-9-775-5111
Mobile : +972-54-893019
E - mail  :yaird@xxxxxxxxxxxxxx


> -----Original Message-----
> From:	R karam [SMTP:rkaram@xxxxxxxxx]
> Sent:	&he; &alef;&vav;ℷ&vav;&samekh;&tet; 10 2000 2:30
> To:	stds-802-3-pwrviamdi@xxxxxxxx
> Subject:	Cross Talk
> Hi Avinom
> your question about cross talk is a valid concern, my 2 cents on this
> are that any noise generated out of the power supply sitting across
> the center taps assuming no capacitors to chassis ground are present
> at each center tap, would cause us EMI headaches way a head of any signal
> interference.
> please keep the filter needed to pass EMI radiated and Conducted testing
> out of the system in mind.also please keep in mind that the center tap
> at lower frequencies has good common mode tolerance per the graphof my
> last presentation (at frequencies below 1MHZ or so the receiver does not
> suffer).  of course I may be missing your point....
> roger