transformer between Ethernet PHY and RJ45

[sraja_23]

ethernet transformer

Why do we need a transformer between Ethernet PHY and RJ45 connector?
Why is this transformer a centre tapped one with centre tap connector to power/GND?

 

[m_pourfathi]

rj45 transformer

well the transformer acts like a filter. It prevents DC signal to be received from and be transmitted to the ethernet line. and also it degrades the noise coming from the line.
For you second question, it's because the signal on ethernet cable is sent differentially. Transmission and receiving line each has a different pair of wires. They are usually shown as Tx+ and Tx- for transmitting and Rx+ and Rx- for receiving. that's why the center of the transformer is connected to the ground.

 

[insrusingh]

ethernet transformer center tap

first of all trnasformer is required for isolation.It will reject the common mode signal and allows differential signal to pass through.
Center tapped because half of the coils ICM/2 will get added and through RC connected through the center pin to ground provides a low impedence path to ground