Why do some USB leads have a "blob" on them?

They are ferrite cores. Do a Google search on "ferrite cores for interference suppression", eg:

It's usually called a "ferrite" or "toroid", and it's there to keep radio frequency noise from being generated/transmitted by the device. Digital data on wires is usually *VERY* "noisy" from a radio viewpoint, and the noise often gets worse as the data speed on the wires increases. At USB speeds, it's possible, if you've got a poorly designed cable or piece of hardware, to completely wipe out nearby radio reception.

If you were to open one up, you'd find that it's usually a black "donut" of material (A soft form of iron, called, appropriately enough, "ferrite") with the wires that make up the cable wrapped through the center, sometimes as a bundle, sometimes each wire individually, then the whole thing is wrapped in a protective plastic cover.

That is a ferrite bead to block some of the common mode noise you might get.

Nope, its to stop the cable radiating.

What I'm interested to know is why do some usb devices have them while others survive quite well without.

The tendency of an electronic device to radiate unwanted radio-frequency energy depends on a number of things, including the frequency of the internal clock circuitry, the power consumption, the PC board layout, the type of case (metal or plastic) and case shielding, the presence or absence of internal RF-blocking components (e.g. small ferrite beads or bypass capacitors), and probably at least a dozen other factors.

Here in the US, the FCC sets limits for how much energy an incidental radiator (e.g. a computer or component thereof) is allowed to emit. In principle, each USB device design is supposed to be tested in a third-party lab, and "certificated" (i.e. shown via testing) to comply with the limits.

So... some USB devices might not need any extra RF blocking on their cables, because their design is inherently a low-speed, low-RF- emitting one. Or, they might not need it becauset the manufacturer put the necessary RF blocking/shielding into the USB device design itself (e.g. a metal case, ferrite beads on the PC board wiring, use of a spread-spectrum oscillator to reduce individual emission spurs, etc.). Or, the manufacturer might be cheating... i.e. certificated one version of the device (with a ferrite-bead cable) and then switched to a less expensive cable design for actual production and sale.

If you're curious, take a look at the bottom/back of your USB device, and find the FCC Part 15 information. You can plug this information into the form at

If you've got a USB device which has no Part 15 certification label, it may be an untested/unauthorized device, and thus potentially illegal to sell here in the U.S.

On Wed, 07 Feb 2007 16:47:33 GMT, Jon D Gave us:

Maybe if you knew what the blob was, and what it is called, you wouldn't have to ask.

It is an RF and EM emissions filter.

The Chinese don't care. The US companies will use them as needed to comply with FCC rules.

It's usually an add-on fix to reduce emissions to a level that'll pass compliance testing. Since it'll cost more to use one than no doughnut - no-one really *wants* to have to use them.

Graham

The Chinese now have to meet IEC regs actually.

Graham

I call them "Balloons" ( they are Baluns ) they keep the intended function of cables to a higher level of performance, within the USB or in some Video Transfer Cables optimal specifications :-)

Funny you mention "The Blob" I just got the latest version of the movie last week....

On Thu, 8 Feb 2007 06:30:13 -0500, snipped-for-privacy@webtv.net (Tick Tock) Gave us:

The Hulk on HD DVD was better.

MassiveProng wrote in news: snipped-for-privacy@4ax.com:

He didn't know, so he asked. Problem? I think not. And you just compounded your own 'cleverness' by answering with a minimal answer that has already been said about TEN times before you showered us with your 'wisdom'. Did you think you saw stupidity, or were you just staring into a mirror by mistake?

Funny you should mention!

Last week I say a "thumb drive" that was meant to plug into the USB outlet. But this was so "bare bones" that it didn't have the metal shell.

Vaguely applicable yes, but not really appropriate which is WHY there are more appropriate groups.

Because that is why groups are segmented, to divert basic questions to groups expressly set up for them. Because there is no end to the basic questions that could be generated, particularly in a more advanced group where the scenarios and terms covered are typically beyond the understanding of those asking basic questions.

In short, you don't grasp how usenet works.

On Sat, 10 Feb 2007 03:18:20 GMT, Wow Gave us:

Once again you proved that you know absolutely nothing about this Usenet forum which you have invaded. Your top posted retarded crap proves that.

You are a foul interloper, at best, little boy. You should bone up on the conventions of the forum you invade BEFORE you go spewing your baby bullshit all over them.

On Sat, 10 Feb 2007 21:52:13 GMT, Wow Gave us:

Still top posting like a little adolescent that doesn't care where he farts, I see.

Get a clue, dipshit.

Nice filter ya got there too. Looks like it works just like your brain does.