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From: Roger Dewhurst on 10 Aug 2010 19:12
Cables which connect USB ports with peripherals such as cameras
frequently have black cylindrical objects on the cables a few
centimetres away from the USB end. These may be about 10mm in diameter
and 20 mm long. Are these things ferrite beads used to eliminate stray
pulses in the cable or something else? If so what?

R
From: Paul on 11 Aug 2010 05:22
Roger Dewhurst wrote:
> Cables which connect USB ports with peripherals such as cameras
> frequently have black cylindrical objects on the cables a few
> centimetres away from the USB end. These may be about 10mm in diameter
> and 20 mm long. Are these things ferrite beads used to eliminate stray
> pulses in the cable or something else? If so what?
>
> R

It's a large ferrite bead, to remove common mode noise from
traveling down the cable.

See the "Ferrite Chokes" section here.

http://en.wikipedia.org/wiki/Sheath_current_filter

A typical reason for using one, is to help an equipment
setup meet FCC part 15 on emissions.

http://www.ce-mag.com/archive/04/Singh.html

"Cables Designed without Ferrites

Common-mode noise occurs in cables when the PCB signal connections
and returns form a common impedance. This type of noise can be
minimized through the use of proper PCB design techniques to
reduce the common-mode impedance or by placing a
ferrite bead around the cable."

HTH,
Paul
From: tony sayer on 11 Aug 2010 06:20
In article <i3smci$dso$3(a)lust.ihug.co.nz>, Roger Dewhurst
<dewhurst(a)wave.co.nz> scribeth thus
>Cables which connect USB ports with peripherals such as cameras
>frequently have black cylindrical objects on the cables a few
>centimetres away from the USB end. These may be about 10mm in diameter
>and 20 mm long. Are these things ferrite beads used to eliminate stray
>pulses in the cable or something else? If so what?
>
>R

Yes your right they are ferrite rings used to prevent spurious RF
signals from entering or exiting the equipment they are connected to.

You'll sometimes notice them on monitor leads as well.

This is done to comply with EMC, ElectroMagnetic Compatibility
regulations....
--
Tony Sayer


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