1 The radiation problem of the cable
The radiation problem of cables is one of the most common problems in engineering. More than 90% of the equipment (mainly equipment containing pulse circuits) can not pass the radiation emission test because of the radiation of the ACSR conductor. There are two mechanisms of cable radiation. One is the differential mode radiation generated by the signal current (differential mode current) loop in the cable, and the other is the common mode current on the wires (including the shielding layer) in the AAC conductor. The radiation of the cable mainly comes from common mode radiation. Common-mode radiation is generated by common-mode current. The loop area of common-mode current is formed by the cable and the ground (or adjacent to other large conductors), so it has a larger loop area and stronger radiation.
How the common mode current is generated is often a problem that many people are confused about. To understand this problem, first make it clear that common-mode voltage is the root cause of common-mode current. Common-mode voltage is the voltage between the cab
and the ground (or other large conductors nearby). Starting from the common-mode voltage, it is easy to find the cause of the common-mode current. Once the cause of a problem is clear, it is not very difficult to solve the problem. The common mode current on the cable is caused by the following points: Common mode current caused by differential mode current leakage. Even if the cable contains a signal return line, it cannot guarantee that 100% of the signal current will return to the signal source from the return line, especially in At higher frequencies, various stray parameters in space provide a third or even more return path for the signal current. Although this common mode current accounts for a small proportion, the radiation cannot be ignored due to the large radiation loop area.
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 the parasitic capacitance at high frequencies has very low impedance. The common mode current is mainly generated by stray capacitance. 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 known 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 and the return line are, the smaller the impedance of the differential mode current loop will be. A typical example is a coaxial cable. Since the return current of the coaxial cable is evenly distributed on the outer sheath, its equivalent current coincides with the axis, so the loop area is zero, the differential mode impedance is close to zero, and almost 100% of the signal current Returning to the signal source from the outer sheath of the coaxial cable, 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, use coaxial cables to communicate to avoid radiation. In fact, this has the same essence as our traditional use of coaxial cables to transmit high-frequency signals to reduce signal loss. Because the signal loss is small, it naturally shows that the leakage component is less, and this part of the leakage is the radiation of the cable.
Common mode current caused by ground noise of the circuit board. The signal ground wire is the return line of the signal. Therefore, there must be a voltage between two points on the ground wire. For high-frequency circuits, these are high-frequency noise voltages, which act as a common-mode voltage to drive the common-mode current on the cable. , Resulting in common mode radiation. The various design methods to reduce the ground wire impedance provided in the circuit board design chapter can be used to reduce the noise on the ground wire, thereby reducing the common mode voltage. A recommended method is to set “clean ground” on the cable port. The so-called clean ground means that there is no circuit that can generate noise on this ground wire, so the local potential on the ground wire is almost equal. If the case is a metal case, connect this piece to the metal case cleanly. Common mode current induced by electromagnetic waves in the chassis.
The chassis is always full of electromagnetic waves. These electromagnetic waves induce common-mode voltage on the cable. In addition, there are some circuits that generate high-frequency electromagnetic fields near the cable ports. There are capacitive coupling and capacitive coupling between these circuits and the cable. Inductive coupling creates a common mode voltage on the cable. Common mode voltage generated by electromagnetic induction. It should be noted that the electromagnetic waves in the chassis are mostly caused by the differential mode radiation of the circuit. In the circuit board design chapter, we discussed the frequency spectrum of the pulse signal differential mode radiation, which shows that the frequency range is very wide. This causes the frequency of the common-mode voltage to be much higher than what we expected.