What is this device?

[BespinReactorShaft]

Some cables e.g. laptop power, data have this thing attached near one end:

http://pics.bbzzdd.com/users/ming2020/ring_thing.jpg

This is on a usb cable. But I've seen more usb cables without it.

I'd really like to know what it's called, what it's for and how it works. As in the case above, is it also an optional device and why?

Thanks.

 

[StevenYoo]

yeah ive always wanted to know what that thing was for

 

[The Sly Syl]

I remember that my gameboy advance link cables had one of those on it, after a little work, i managed to snap it apart and take it off. It was a little heavy to be purely plastic, but i never could figure out what it was..

So I just snapped it back on and stopped wondering.

I'm thinking it may be magnets of some sort, to keep the power shielded near the ends?

 

[PottedMeat]

It is a ferrite core - used for suppressing electromagnetic interference. Basically it keeps noise from interfering with the signal. At signal frequencies ( which you want to pass ) the core is low impedance and allows the signal to pass. At high frequencies which may interfere with signal reception the ferrite presents a high impedance and attenuates the noise.

http://www.fair-rite.com/newfair/pdf/CUP%20Paper.pdf

 

[BespinReactorShaft]

What dictates whether a manufacturer includes a ferrite core with a particular cable? Is it dependent on the peripheral type/specs, regulatory requirements that affect certain countries/manufacturers but not others, or simply how it affects profit margins?

 

[Peter]

Peripheral cables are one major path for EMI emissions out of a computer. Those ferrit beads catch them. Note this is not about protecting the signal that is supposed to be on the cable, it is about keeping those that shouldn't from radiating out into the open.