Ethernet transformer vs. RJ-45

Hello, I have seen in several layout guidelines that the Ethernet transformer should be placed as close to the RJ-45 connector as possible, while the close placement of a PHY to the transformer is not as critical. Why is that? It seems counter-intuitive: I would think that the PHY should as close to the transformer as possible, while the lines between the transformer and the RJ-45 are just differential pairs and, thus, are not much different from the CAT5 cable. So, why is the close placement of the transformer to the RJ-45 is more critical than the close placement of the PHY to the transformer?

Thank you, Eric.

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This is just a guess at a theory...

Perhaps it is because on the PHY side, the lines are a differential pair referenced to the board's ground, while after the transformer there is no common reference. There could therefore be a significant voltage difference between the transformed pair and the board's ground, or significant common-mode noise.

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David Brown

My theory is....

All fast signals weather differental or not should be short as possible.

The Micro, Ethernet PHY, Magnetics and R45 should be as short as possible.


Reply to
Joe G (Home)


And then you plug a 50-foot patch cord into the RJ45...

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I think a general rule about impedance controlled transmission lines is that you want to have any unavoidable impedance "bumps" done with over as short a distance as possible. Thus the path from the transformer, over the pcb, and into the RJ45 should be short, while the smooth impedance cable can be longer.

Basically, you want the electrons to go over the speed bumps before they have had a chance to accelerate to full speed along the straight cable.

(Perhaps someone who actually knows the answer could tell us.)

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David Brown

"ericbogomolny" ...

The lines between RJ45 and transformer need 1500V isolation to the rest of the world, the lines to the PHY do not. Just think about running long 1500V isolated lines over a crowded PCB...


Reply to
Arie de Muynck

Correct - another similar issue is 3 phase. If one end of the ethernet was powered up by one phase, and the other receiving end was on a different phase, you can get minute leakage and 415 volts. You don't want that all over your system.

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