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Re: [802.3_4PPOE] Regarding adding perfectly balanced resistance to a cable measurement


From: Jeff Heath [mailto:jheath@xxxxxxxxxx]
Sent: Tuesday, July 01, 2014 12:54 PM
To: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: Regarding adding perfectly balanced resistance to a cable measurement


Hi Yair,


You have invested a lot of effort in using percentage resistance imbalance.  I believe your explanation today was:


1.    Use simulation of resistance and voltage offsets to create imbalance current numbers

2.    Convert this data to a resistance imbalance percentage for the worst case of the simulation(s)

3.    Define the system components in terms of percentage


Using this technique you have run into a problem.  The cable resistance is not well suited to using a percentage because it depends on a function that has discontinuities in it.  These jumps in percentage are due mainly to the addition of connectors.  Since this ‘breaks’ your desire to have a system components that are defined by percentages, you have decided to add some resistance, perfectly balanced resistance in sufficient quantity to a cable measurement so that the percentage falls below a number you have chosen (I believe that number for the cable is 7% right now in your presentation).


This new technique has several problems:

1.    It does not represent the actual cable

2.    You suggest that the resistance you are adding is already present in the PSE and PD but

a.    These resistances are not in the channel

b.    These resistances are not balanced


Also, during the meeting it was brought up that some of the use cases you created for the channel should be discussed because they are extremely unlikely.  To that end they should be discussed and a consensus reached.  One of your arguments was that anybody could do anything which is not a practical approach to solving imbalance if it is not at all likely.


As an example of this, I believe you  have a 0.25m cable with 1 connector which was thought to be very unlikely.  If you really insist that anything should be considered, why not include a 0.25m cable with 4 connectors? 


The channel is there and we should define and agree on the use cases we are to consider in this ad-hoc and accept the results in the current imbalance they create when simulated with the PSE and PD PIs and report the maximum numbers.




Jeff Heath
Design Center Manager

Description: Linear Technology Corporation




402 East Carrillo Street, Suite D


Santa Barbara, California 93101