What are these in USB and other cables?

[kumarkumar]

Take a look at these photos of USB and other cables

You will notice a thick plastic cylinder on the cables. (somtimes with the cable making one twist around it...2nd and 4th pic)

What is that?? What is the purpose of that??

Edited April 26, 2007 by kumarkumar

[jcarle]

It's to help reduce EMI interference.

[phkninja]

Just an addition. They are only used on cheaper cable (e.g. non-shielded) to stop outside interference with the data on the cable, if shielded cable was used the magnets would no longer be required

[kumarkumar]

Thanks a lot to jcarle & phkninja for the quick answers.

The link explained that nicely in layman terms.

What I was wondering from the anwser in the link is ........say I have a 3 foot long "straight" cable carrying signals through it. Now it would emit radially as well as absorb EM waves all along the length of the cable.

Now the ferrite core acts as a flux sponge. So just a small core at one end of the cable absorbs all the EM waves coming from the whole length of the cable??

[phkninja]

yes. it cancels the effect of the extra EM. its only needed close to the connector to cancel at the last minute so the EM isnt added to the siganl at the computer etc.

You will also find that usually only long cables have these "extra" bits, as the lenght of the cable directly effets the amount of EM and signals from other sources.