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On many monitor or chargers, there is a bold part:

Monitor

First of all, what is it called?

Then, what does it do?

Does a cable without this part have any advantages as we see nowadays many chargers don't have this part anymore?

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It is called a common mode choke. It is a toroidal piece of ferrite with the wires of the cable passing thru the hole.

The cable and the ferrite form a small-valued inductor, but only to the sum of the currents in the cable. This inductor increases the impedance at high frequencies. The purpose is usually to attenuate these frequencies so that RF noise created in the monitor doesn't get onto the cable. The cable can act as a antenna and radiate the signal, and conduct it into whatever the other end of the cable is plugged into.

This choke works both ways. It also attenuates high frequency noise coming into the monitor.

Note that this affect only works on the total sum of the currents in the cable. Differential signals that are properly balanced have the same current going out on one wire as coming in on another. The total current going thru the hole in the ferrite as a result of such a differential signal is therefore zero. This means no magnetic field is created, and the ferrite presents no increase of impedance to the differential signal.

Unintended noise signals coming out of the monitor are mostly common mode, since they are superimposed on all the conductors of the cable. These do cause a net current to be seen by the ferrite, which causes a magnetic field, which increases the impedance to those signals. Currents all going in one direction are called common mode, which is why this is called a common mode choke.

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  • \$\begingroup\$ Thanks very much. Still I would like to know what is the disadvantage of using ferrite bead? I have seen some one advertising that their product excludes ferrite bead. \$\endgroup\$
    – zahmati
    May 27, 2015 at 0:11
  • \$\begingroup\$ @zahmati: The disadvantage is basically cost. You also then have to use their special cable for the monitor to stay in compliance, but the manufacturer doesn't case what you do as long as what they ship is compliant. Sometimes there isn't room in the product for the ferrite. Other times they discovered the external ferrite was needed to pass emissions, and specifying such a cable is cheaper than re-spinning the design, or at least quick to market. After-market cables often just include the ferrite, sometimes at both ends. \$\endgroup\$ May 27, 2015 at 12:07
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It's a (relatively large) ferrite bead, installed by the manufacturer on cables when the electronics at one or both ends of the end of the cable didn't pass EMC compliance testing (required for most commercial electronic products), and it was deemed cheaper to add the ferrite to the cable than to actually go back and do the job properly and do the filtering on the PCB.

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