On a network interface, the network transformer will perform differential mode signal transmission, electrical voltage isolation, impedance matching, waveform repair, signal clutter suppression and more.

Signal data transmission: The network transformer filters the differential signal sent by the PHY with the differential mode coupled coil to enhance the signal, and is coupled to the other end of the connected network cable of different levels by the conversion of the electromagnetic field to achieve the purpose of transmitting data.

Electrical voltage isolation: The transformer function itself is used to isolate different levels between different network devices connected by the network cable to prevent different voltages from being transmitted through the network cable to damage the device.

In addition, data mercury can also provide some lightning protection for the device.

Network transformer three major functions

First, electrical isolation

The signal level generated by any CMOS process chip is always greater than 0V (depending on the chip's process and design requirements). When the PHY output signal is sent to 100 meters or more, there will be a large loss of DC component. . 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.

Then the equipment grounding method is different. The grid environment will cause the 0V levels of the two parties to be inconsistent, so the signal will be transmitted from A to B. Since the 0V level of the A device is different from the 0V level of the B point, this may cause a large Current flows from a device with a high potential to a device with a low potential.

The network transformer couples the differential signal sent by the PHY with a differential mode coupled coil to enhance the signal and is coupled to the other end of the connected network line by electromagnetic field conversion. This not only makes the physical connection between the network cable and the PHY but also transmits the signal, blocks the DC component in the signal, and can also transmit data in devices with different 0V levels.

The network transformer itself is designed to withstand voltages from 2KV to 3KV. It also plays a role in lightning protection. Some friends' network equipment is easily burnt out during thunderstorms. Most of them are caused by unreasonable PCB design, and most of them burned the interface of the equipment. Few chips were burnt, that is, the transformer played a protective role.

The isolation transformer can meet the insulation requirements of IEEE802.3, but it can not suppress EMI.

Second, common mode suppression

Each of the wires in the twisted pair is intertwined with a double spiral structure. The magnetic field generated by the current flowing through each wire is constrained by the spiral shape. The direction of current flowing through each of the wires in the twisted pair determines the degree to which each pair of wires emits noise. The degree of emission caused by the differential mode and common mode current flowing on 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.

1. Differential mode signal in twisted pair

For a differential mode signal, its current on each conductor is transmitted in a reverse direction over a pair of conductors. If the pair of wires are evenly wound, these opposite currents produce an equal, reverse-polarized magnetic field that causes their outputs to cancel each other out.

2. Common mode signal in twisted pair

The common mode current flows in the same direction on the two wires and returns to the ground via the parasitic capacitance Cp. In this case, the current produces magnetic fields of equal magnitude and polarity, and their outputs do not cancel each other out.

Third, common mode, differential mode noise and its EMC

There are two types of noise on the cable: radiated noise and conducted noise generated from the power cable and signal cable. These two categories are divided into common mode noise and differential mode noise. Differential mode conducted noise is the noise current generated by the noise voltage inside the electronic device and the same path as the signal current or the power supply current, as shown in Figure 4. The way to reduce this noise is to reduce the high frequency noise by connecting capacitors in series or in parallel on the signal and power lines or by using a capacitor and inductor to form a low-pass filter.

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 area of the current and current loop. Therefore, the method of reducing this radiation is to add an LC low-pass filter at the signal input to prevent noise current from flowing into the cable; use a shielded cable or a flat cable to transmit return current and signal current in adjacent wires to make the loop The area is reduced.

Common mode conducted noise is generated by the noise current flowing between the earth and the cable, driven by the noise voltage inside the device, through the parasitic capacitance between the earth and the device.

The method of reducing the common mode conduction noise is to connect a common mode choke coil in a signal line or a power line, connect a capacitor between the ground and the wire, form an LC filter for filtering, and filter common mode conducted noise.