The resistance of the nickel zinc magnetic ring to high-frequency components in the circuit is about ten to several hundred Ω, so its role in high impedance circuits is not obvious. On the contrary, it will be very effective when used in low impedance circuits such as power distribution, power supply, or RF circuits.

The nickel zinc magnetic ring is applied in the high-frequency range (greater than 10MHz), and the inductance remains very small, while the impedance is large, which causes the energy of the high-frequency signal to be converted into heat and dissipated when passing through the magnetic material. This hinders the passage of the high-frequency signal and suppresses the interference of the high-frequency signal. Usually, the suppression frequency range is related to the ferrite suppression component, and the higher the magnetic permeability, the lower the suppression frequency, the larger the volume of the ferrite, and the better the suppression effect, When the volume is fixed, long and thin ones have better inhibitory effects than short and thick ones. The smaller the internal force, the better the inhibitory effect.

The application of nickel zinc magnetic rings has different impedance characteristics at different frequencies. Generally, the impedance is very small at low frequencies, but sharply increases at high frequencies. The higher the signal frequency, the easier the magnetic field is to radiate out. Generally, signal lines are not shielded, such as the CAN bus I use now. These signal lines become perfect antennas, which continuously receive high-frequency signals from the surrounding environment. The superposition of these signals changes the actual signal to be transmitted, The magnetic ring can effectively pass through useful signals while suppressing high-frequency interference signals.

When using a nickel zinc magnetic ring to suppress common mode signal interference, the magnetic ring can simultaneously pass the signal (connected to a differential signal line) or power line (positive and negative lines) through the magnetic ring. To increase the effect, the magnetic ring can be symmetrically wound several times to increase the inductance and enhance the absorption effect of the common mode signal. However, it does not affect the differential signal. The component should be installed near the interference source. For input and output circuits, It should be as close as possible to the inlet and outlet of the shielding box.

The nickel zinc magnetic ring application passes the entire bundle of cables through a ferrite magnetic ring to form a common mode choke. As needed, the cable can also be wound several turns on the magnetic ring. The more turns, the better the suppression effect on interference with lower frequencies, while the weaker the suppression effect on noise with higher frequencies. In practical engineering, the number of turns of the magnetic ring should be adjusted based on the frequency characteristics of the interference current. Usually, when the frequency band of the interference signal is wide, two magnetic rings can be placed on the cable, each with a different number of turns, which can simultaneously suppress high-frequency interference and low-frequency interference. From the mechanism of the action of the common mode choke, the larger its impedance, the more obvious its interference suppression effect. The impedance of the common mode choke 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 this is not the case in reality, as there are parasitic capacitors on the actual magnetic ring, which exist in parallel with the inductance. When encountering high-frequency interference signals, the capacitive reactance of the capacitor is small, which shortens the inductance of the magnetic ring and makes the common mode choke ineffective.

According to the frequency characteristics of interference signals, nickel zinc magnetic rings or manganese zinc ferrite can be selected, with the former having better high-frequency characteristics than the latter. The magnetic permeability of manganese zinc ferrite ranges from thousands to tens of thousands, while the nickel zinc magnetic ring ranges from hundreds to thousands. The higher the magnetic permeability of ferrite, the greater its impedance at low frequencies and the smaller its impedance at high frequencies. Therefore, when suppressing high-frequency interference, it is advisable to use nickel zinc magnetic rings for the application of magnetic rings; On the contrary, manganese zinc ferrite is used. Alternatively, both manganese zinc and nickel zinc ferrite can be sheathed on the same bundle of cables, which can suppress interference in a wider frequency band.

Why is a magnetic ring added to a USB cable? The main purpose of the shielding layer is to shield external interference with signals on the line and avoid data mutation during transmission; The anti-interference magnetic ring is used to shield the interference signals generated from the inside of the circuit from being transmitted to other devices, which is clearly different from the inside and outside.

Secondly, the interference channels of electromagnetic interference signals mainly include the following:

1. Space radiation, which is the most common;

2. Conduction, such as conducted disturbances on power lines;

3. Capacitive/inductive coupling, such as crosstalk between adjacent digital lines;

1、 Shielding only works against interference from space radiation, so if the interference is the second or third type, shielding may not be effective;

2、 Even if the interference belongs to space radiation, it still depends on the shielding size and interference signal wavelength. There will always be signal shielding

The so-called exposed window, assuming that the size of the window is greater than the wavelength of the interference signal, or is in the same order of magnitude as the wavelength of the interference signal, then the interference signal can also enter the shielding body. The shielding layer may not be able to shield all interference. So the reason why a USB cable still needs an anti-interference magnetic ring even if it has a shielding layer is to ensure that there is no interference inside the line.

Returning to the USB cable again, have you ever been exposed to surge phenomena? If you have been exposed to surge phenomena, you must understand. Problems caused by interference from signals or circuits. If this occurs, it can affect normal transmission at a light level, or burn out USB circuits and USB devices at a heavy level. The length of the USB cable will also be affected. The longer the USB cable length, the more likely it is to solve the problem by adding a shielding layer and anti-interference RH series magnetic rings or MLF and MRC series products.