The lump on the cable is a tubular ferrite either moulded inside of plastic or encased in a snap-on plastic case. These ferrites are placed on the cable of various devices, as the others have already indicated, to control RF noise. It is my experience that these were usually determined to be necessary when a particular manufacturer took their product to an FCC or TUV type lab to perform CE or FCC verification testing for emissions and found a lot of RF garbage coming out of their product. The manufacturers of these ferrites leave free sample kits of the ferrites at the test labs and the product tester will often clip one onto cables as a quick test to see of the equipment under test would pass. Sometimes it is found to help to pass the test!!

Once the test reports are written showing the ferrite on the cable the manufacturer tends to just leave them on there for the rest of the product life.

I have found however that with proper equipment design these ferrites on cables are not necessary !!. On top of all that the ferrites, being on the outside of the equipment enclosure, are far from being in an ideal location.

So please note....whenever you see one of these things on a cable to a piece of equipment...you will know that the associated peice of equipment barely passed emissions limits at the test lab and a free sample of ferrite seemed to help with the emissions to a certain degree!!! But also note that same said piece of equipment would probably NOT be the best to look inside of for an example of how to design for minimum emissions.

Michael Karas