The cause of the common-mode current on the cable

  1. Common mode current caused by differential mode current leakage.

  Even if the cable contains a signal loop, there is no guarantee that 100% of the signal current will return to the signal source from the loop, especially at higher frequencies, because various stray parameters provide a third or even more return path for the signal current. Path.Although this common-mode current accounts for a small proportion, due to the large area of the radiation loop, the radiation cannot be ignored.Do not try to reduce the common-mode current by “disconnecting” the circuit from the ground (disconnecting the ground wire between the circuit board and the chassis, or disconnecting the ground wire between the chassis and the ground), thereby reducing the common-mode radiation.Disconnecting the circuit from the ground can only reduce the common-mode current at low frequencies, and the path formed by parasitic capacitance at high frequencies has a small impedance.。

  2. The common-mode current is mainly generated by stray capacitors.

  Of course, if the problem of common-mode radiation mainly occurs at low frequencies, disconnecting the circuit board or chassis from the ground will have a certain effect.From the mechanism of common-mode current generation, it can be seen that an effective way to reduce this common-mode current is to reduce the impedance of the differential-mode loop, thereby prompting most of the signal current to return from the signal ground.Generally, the closer the signal line is to the loop, the smaller the impedance of the differential mode current loop.A typical example is a coaxial cable. Since the return current of the coaxial cable is evenly distributed on the outer skin, its equivalent current is combined with the axis, so the loop area is zero, the differential mode impedance is close to zero, and almost 100% of the signal current is returned from the outer skin of the coaxial cable to the signal source, and the common-mode current is almost zero, so the common-mode radiation is very small.On the other hand, since the area of the differential mode current loop is almost zero and the differential mode radiation is also very small, the radiation of the coaxial cable is very small.For high-frequency signals, the transmission of coaxial cables can avoid radiation.In fact, this has the same essence as our traditional purpose of transmitting high-frequency signals with coaxial cables to reduce signal loss.Because the loss of the signal is small, it naturally means that there are fewer components of the leakage, and this part of the leakage is the radiation of the cable.

  3. The common-mode current caused by the ground noise of the circuit board.

  The signal ground wire is the return line of the signal. Therefore, there must be a voltage between the two points on the ground wire. For high-frequency circuits, these are the high-frequency noise voltages. As a common-mode voltage, it drives the common-mode current on the cable, resulting in common-mode radiation.The various design methods provided in the chapter on circuit board design to reduce the impedance of the ground wire can be used to reduce the noise on the ground wire and thereby reduce the common-mode voltage.One recommended method is to set “cleanly" on the cable port.The so-called clean ground means that there is no noise-generating circuit on this ground wire, so the local potential on the ground wire is almost equal.If the chassis is a metal chassis, connect this piece cleanly with the metal chassis.Recommended products: Mining cable

  4. The common-mode current caused by the spatial induction of electromagnetic waves in the chassis.

  The chassis is always full of electromagnetic waves. These electromagnetic waves will induce a common-mode voltage on the cable. In addition, there will be some circuits that generate high-frequency electromagnetic fields near the cable port. There is capacitive coupling and inductive coupling between these circuits and the cable, forming a common-mode voltage on the cable.The common-mode voltage generated by electromagnetic induction.It should be noted that most of the electromagnetic waves in the chassis are caused by the differential mode radiation of the circuit.