magnetics/transformer inside a RJ45 connector

Thread Starter

bug13

Joined Feb 13, 2012
1,995
Hi team,

Just trying to understand the function of a magnetics/transformer for inside a JR45 connector in an ethernet application.

The original circuit is here, page 6.

Ref to A: What is R1 and R2, R3 and R4 are for, are they 50ohm termination resistor?? Why the center tap need to pull to high (3.3V)?
Ref to B: Is that for isolation purpose?
Ref to C: Is that a common mode choke??
Ref to D: they they termination resistor again?

Thanks team!
Screenshot 2021-03-22 162548.jpg
 

ronsimpson

Joined Oct 7, 2019
1,471
The project will work with out any transformers but you must have the grounds at the same level and have no noise.
The transformers on the left side are to isolate grounds. Probably the signal is coming from another room and the grounds are not the same. The transformers will isolate to 2000 volts. They pass high frequency data and reject DC and 60hz power line frequencies.
The transformers on the right are common mode filters. They are built to pass data on pins 6 & 3 but reject noise.
 

Thread Starter

bug13

Joined Feb 13, 2012
1,995
The project will work with out any transformers but you must have the grounds at the same level and have no noise.
The transformers on the left side are to isolate grounds. Probably the signal is coming from another room and the grounds are not the same. The transformers will isolate to 2000 volts. They pass high frequency data and reject DC and 60hz power line frequencies.
The transformers on the right are common mode filters. They are built to pass data on pins 6 & 3 but reject noise.
How about the resistors, what do they do?
 

sparky 1

Joined Nov 3, 2018
579
The use of isolation transformers might be a design choice in the example shown.
ESD and noise reduction might be one concern in some machine to machine RX, TX connections
using encapsulated array of small ferrite transformers can be an easy fix some like optical isolation.
 
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