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Warum tun Sie, Gleichtaktstrom treten auf Drähten und Kabeln auf?

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Warum tun Sie, Gleichtaktstrom treten auf Drähten und Kabeln auf?

August 20, 2021
Aktueller Firmenfall über Warum tun Sie, Gleichtaktstrom treten auf Drähten und Kabeln auf?

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

Even if the cable contains the signal return line, it cannot guarantee that the signal current will return to the signal source from the return line 100%. Especially in the high frequency occasions, the various stray parameters of the space provide the third or even more signal current for the signal current. The return path. 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 parasitic capacitance at high frequencies has very low impedance.

 

2. The common mode current is mainly produced 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. It can be known from the mechanism of common mode current generation 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 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.

 

3. 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 are used as common-mode voltages to drive the common-mode current on the cable. , Resulting in common mode radiation. The various design methods for reducing 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 a "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 potentials on the ground wire are almost equal. If the case is a metal case, connect this piece to the metal case cleanly.

 

4. the common mode current induced by the electromagnetic wave space 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.

 

If you have any other questions, please consult Chengdu Nuoxing Cable Co., Ltd. for details.

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