A, electrical isolation

The signal level is always greater than the 0V of any CMOS process chip's work (depending on the chip fabrication and design requirements), the output signal of the PHY to 100 meters or more places will have great losses in the DC component. But if the external line directly and chip connected words, electromagnetic induction (thunder) and electrostatic, damage is very easy to cause the chip.

Different is the equipment grounding method, different grid environment can lead to 0V level both inconsistent, so that the signal from A to B, because the 0V level 0V level and B A devices are not the same, this may lead to large current from high to low voltage equipment electric equipment.

Network transformer coil coupling filter PHY to send out the differential signal using differential mode to enhance the signal, and through the conversion of the electromagnetic field coupled to another end is connected to the cable. Also the effect of lightning protection.

Network equipment some friends easily in the thunderstorm weather when burned, mostly PCB unreasonable design, mostly destroyed equipment interface, there are few chips burned, is the transformer plays a protective role. Isolation transformer can meet the insulation requirements of IEEE802.3, but could not inhibit EMI. Two, common mode rejection

Each wire in twisted pair cable is a double spiral structure entwined.

Each wire in twisted pair cable is a double spiral structure entwined.

Control current through each wire magnetic field produced by the spiral. Each wire current direction through the twisted pair cable, decided to launch the noise level of each wire. Emission level in each pair of wires through the differential mode and common mode current is caused by different, differential mode current caused by the noise emission is smaller, so the noise is mainly decided by the common mode current. The differential signal pair of 1.

The differential signal pair of 1.

The differential mode signal, in every wire current is in the opposite direction in a pair of wire transfer. If the wire is uniformly wound, the reverse current will produce equal, the reverse polarized magnetic field, so that its output offset each other.

The twisted pair of common mode signal

Common mode currents flowing in the same direction in the two wires, and through the parasitic capacitance Cp to return. In this case, current equal to the size of the same polarity magnetic field, their output cannot cancel each other out. As shown in Figure 6, the common mode current produces a magnetic field on the surface of wire antenna, as it.

Three, common mode, differential mode noise and EMC

Cable noise radiation noise and transmission noise generated from the power cable and signal cable two categories. These two categories are divided into common mode noise and differential mode noise two. Differential mode conducted noise is noise current electronic equipment within the voltage noise generated current and signal current or power on the same path, as shown in figure 4. Method for reducing the noise is the power and signal line line in tandem differential mode choke, the shunt capacitance or inductance and capacitance low-pass filter, to reduce the high frequency noise.

The strength of the electric field and the noise generated by the cable to the observation point is inversely proportional to the distance, is proportional to the square of the frequency, is proportional to the current and current loop area. Therefore, methods to reduce the radiation with LC low pass filter to prevent noise in the signal input current into the cable; the use of shielded cable or flat cable, transmission return current and current signal in adjacent wires, the loop area decreased.

The common mode conducted noise is driving noise voltage in the equipment, through the parasitic capacitance between the earth and the equipment, the noise current to flow between the earth and the cable production.

Methods to reduce the common mode conducted noise in the signal line or a power line in series with a common mode choke, between the ground and the wire shunt capacitors, LC filter is used to filter, common mode conducted noise.