Passing the entire bundle of cables through a ferrite magnetic ring constitutes a common mode choke coil. According to requirements, the cable can also be wound several turns on the magnetic ring. The more the number of turns, the better the suppression effect on lower frequency interference, and the weaker the suppression effect on higher frequency noise. In actual engineering, the number of turns of the magnetic ring should be adjusted according to the frequency characteristics of the interference current. Usually when the frequency band of the interference signal is relatively wide, two magnetic rings can be put on the cable, and each magnetic ring has a different number of turns, so that high-frequency interference and low-frequency interference can be suppressed at the same time. From the point of view of the mechanism of the common mode choke coil, the larger the impedance, the more obvious the interference suppression effect. The impedance of the common mode choke coil comes from the common mode inductance Lcm=jwLcm. It is not difficult to see from the formula that for a certain frequency of noise, the larger the inductance of the magnetic ring, the better. But the actual situation is not the case, because there is a parasitic capacitance on the actual magnetic ring, and its way of existence is in parallel with the inductance. When encountering high-frequency interference signals, the capacitive reactance of the capacitor is small, which short-circuits the inductance of the magnetic ring, thereby making the common mode choke coil useless.
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