Transformer function

1. electrical isolation
The signal level generated by any CMOS process chip is always greater than 0V (depending on the process and design requirements of the chip), and the PHY output signal will have a large DC component loss when sent to 100 meters or even longer. Moreover, if the external network cable is directly connected to the chip, electromagnetic induction (thunder) and static electricity can easily cause damage to the chip.
Another is that different grounding methods of equipment and different power grid environments will lead to inconsistent 0V levels of both parties, thus signals are transmitted from A to B. Since the 0V level of equipment A is different from that of point B, a large current may flow from equipment with high potential to equipment with low potential.
The network transformer filters the differential signal sent by the PHY with differential mode coupling coil coupling to enhance the signal, and couples it to the other end of the connected network cable through electromagnetic field conversion. In this way, there is no physical connection between the network cable and PHY and the signal is transferred instead, the DC component in the signal is cut off, and data can be transmitted in devices with different 0V levels.
The network transformer itself is designed to withstand the voltage of 2KV ~ 3KV. Also played a role in lightning protection. Some friends of the network equipment in the thunderstorm weather is easy to burn out, mostly caused by unreasonable PCB design, and mostly burned the interface of the equipment, few chips were burned, is the transformer played a protective role.
The isolation transformer can meet the insulation requirements of IEEE 802.3, but cannot suppress EMI.
2. common mode rejection
Each conductor in the twisted pair is intertwined in a double helix configuration. The magnetic field generated by the current flowing through each wire is constrained by a spiral shape. The direction of the current flowing through each wire in the twisted pair determines the degree of noise emitted by each pair of wires. The degree of emission caused by the differential mode and common mode current flowing through each pair of wires is different, and the noise emission caused by the differential mode current is small, so the noise is mainly determined by the common mode current.
The differential mode signal in the twisted pair
For a differential mode signal, the current on each wire is transmitted in opposite directions on a pair of wires. If the pair of wires are uniformly wound, these opposing currents will produce equal and oppositely polarized magnetic fields, causing their outputs to cancel each other out.
The common mode signal in twisted pair
The common mode current flows in the same direction on both wires and returns to ground through the parasitic capacitance Cp. In this case, the currents produce magnetic fields of equal magnitude and polarity, and their outputs cannot cancel each other out. The common mode current generates an electromagnetic field on the surface of the pair of strands, which acts like an antenna.
3. common mode and differential mode noise and its EMC
There are two types of noise on the cable: radiated noise and conducted noise generated from power cables and signal cables. These two categories are divided into common mode noise and differential mode noise. The differential mode conduction noise is the noise current generated by the noise voltage inside the electronic equipment in the same path as the signal current or the power supply current. The way to reduce this noise is to series differential mode chokes, parallel capacitors or low-pass filters with capacitors and inductors on the signal lines and power lines to reduce high-frequency noise.
The intensity of the electric field generated by this noise is inversely proportional to the distance from the cable to the observation point, proportional to the square of the frequency, and proportional to the current and the area of the current loop. Therefore, the method to reduce this radiation is to add an LC low-pass filter to the signal input terminal to prevent noise current from flowing into the cable; using a shielded cable or a flat cable to transmit the return current and the signal current in the adjacent wires, so that the loop area is reduced.
Common mode conduction noise is driven by the noise voltage in the device, through the parasitic capacitance between the earth and the device, the noise current flowing between the earth and the cable.
The method to reduce the common mode conduction noise is to connect the common mode choke coil in series in the signal line or power line, connect the capacitor between the ground and the wire, and form an LC filter to filter the common mode conduction noise.