usb cables

  • Thread starter Thread starter Juve
  • Start date Start date
J

Juve

HI,

Some USB cables have got a cylinder shaped "thing" on it. What is the
use of it and can cables with it be interchanged with cables that
haven't got it?

OR
 
Juve said:
HI,

Some USB cables have got a cylinder shaped "thing" on it. What is the
use of it and can cables with it be interchanged with cables that
haven't got it?

OR
Click to expand...

Ferrites are used for noise control, but for high frequency digital
signals, they cause distortion. If I was buying USB cables,
I'd buy ones with a shield foil around the outside of them,
but I would not get one with a ferrite ring.

You'll notice on page 14 of this document, they talk about
noise control devices inside the computer, but not about
snapping a ferrite ring around the USB cable. I suspect the
eye diagram might be compromised for USB2, if a ring was
present. (You'd need a ferrite which passed 400MHz signals
and harmonics, without attenuation.) The ring might work OK
for USB 1.1 applications, or if the cable was going to a USB
keyboard. (Check to see if the cable was bundled with a
device that only operated at USB 1.1 .)

http://www.murata.com/catalog/c35e.pdf

This cable is transparent, and you can see the shield around the
conductors inside the cable. The cable you buy, doesn't have to
be transparent, but it does make it easier to see it is shielded.
If the advert says it works for USB2, then it should be good
for any application.

http://c1.neweggimages.com/NeweggImage/productimage/12-200-170-03.jpg

Paul
 
Juve wrote:
Click to expand...
Ferrites are used for noise control, but for high frequency digital
signals, they cause distortion. If I was buying USB cables,
I'd buy ones with a shield foil around the outside of them,
but I would not get one with a ferrite ring.

You'll notice on page 14 of this document, they talk about
noise control devices inside the computer, but not about
snapping a ferrite ring around the USB cable. I suspect the
eye diagram might be compromised for USB2, if a ring was
present. (You'd need a ferrite which passed 400MHz signals
and harmonics, without attenuation.)
Click to expand...

I think you are missing a point here.

Your USB data travels within both a twisted pair and a shielded
environment. It shouldn't --and most probably won't-- 'see' any
ferrites around the cable's perimeter.
The MuRata paper you refer to discusses the use of ferrites
inside the signal paths, not clip-on ferrites around the cable.

On the other hand: the shield on any cable attached to a PC acts
as an antenna. In both ways. It picks up noise from the
environment and it spreads noise generated by the PC.
Attaching a proper ferrite clip-on will diminish this antenna
behavour. Also in both ways, if all goes well :-)
 
Peter Able said:
By the way - you may have noticed that USB cables sold on their own tend
not
to be fitted with ferrites, but are when the cable is part of an item -
e.g.
an external USB hard drive. There's evidence for the above explanation!
Click to expand...

My little HP digicam came with a ferrite that I had to install by winding
the USB cord around it. I've wondered if that was just an afterthought, due
to a regulatory stipulation they didn't plan for.

It was kind of funny having instructions for winding the USB cord around the
bobbin.

Jon
 
Paul said:
Ferrites are used for noise control, but for high frequency digital
signals, they cause distortion. If I was buying USB cables,
I'd buy ones with a shield foil around the outside of them,
but I would not get one with a ferrite ring.

You'll notice on page 14 of this document, they talk about
noise control devices inside the computer, but not about
snapping a ferrite ring around the USB cable. I suspect the
eye diagram might be compromised for USB2, if a ring was
present. (You'd need a ferrite which passed 400MHz signals
and harmonics, without attenuation.) The ring might work OK
for USB 1.1 applications, or if the cable was going to a USB
keyboard. (Check to see if the cable was bundled with a
device that only operated at USB 1.1 .)

http://www.murata.com/catalog/c35e.pdf

This cable is transparent, and you can see the shield around the
conductors inside the cable. The cable you buy, doesn't have to
be transparent, but it does make it easier to see it is shielded.
If the advert says it works for USB2, then it should be good
for any application.

http://c1.neweggimages.com/NeweggImage/productimage/12-200-170-03.jpg

Paul
Click to expand...

Ferrite rings act as common mode inductors, so there's no extra inductance
introduced in the signal path as it cancels out. There's only inductance
there for when the same signal is on both wires, i.e. externally picked up
radiation that affects both wires the same, and the ferrite ring will
effectively reduce this because this signal has to pass through the extra
inductance. I've proved this to myself once using a pulse generator and
100MHz scope, even a huge ferrite doughnut ring with the cable twisted 10
turns round it doesn't affect the real signal, but noise pickup from a
nearby switch mode PSU is reduced dramatically.
 
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