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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Terminations
We mentioned that the impedance of the twisted-pair transmission line is 100 Ω. In the topmost part of Fig. 2.10, on the left (PHY) side of the transformer, we have two 50 Ω in series between the two wires of the twisted pair. So we get a total of 100 Ω. We know that any impedance reflects across a transformer boundary according to the square of the turns ratio. The drive transformer in a typical case has a turns ratio of 1:1, so the 100 Ω on the PHY side appears on the line side as 100 Ω, and that is the appropriate termination for a 100 Ω line (it matches the impedance of the line as desired). Note that if there is a common-mode disturbance, the center node of the two 50-Ω resistors will not remain at a fixed potential, but will ...

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