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

  • Thread starter Thread starter Jon D
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Jon D

Why do some of my USB leads (for example those which came with my
dictation machine) have a largish cylindrical plastic "blob" on them?

The blob is about an inch long and half an inch is diameter.
 
Jon D <[email protected]> said:
Why do some of my USB leads (for example those which came with my
dictation machine) have a largish cylindrical plastic "blob" on them?

The blob is about an inch long and half an inch is diameter.
Click to expand...

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.
 
Why do some of my USB leads (for example those which came with my
dictation machine) have a largish cylindrical plastic "blob" on them?

The blob is about an inch long and half an inch is diameter.
Click to expand...
That is a ferrite bead to block some of the common mode noise you
might get.
 
Jon D said:
Why do some of my USB leads (for example those which came with my
dictation machine) have a largish cylindrical plastic "blob" on them?

The blob is about an inch long and half an inch is diameter.
Click to expand...

What I'm interested to know is why do some usb devices have them while
others survive quite well without.
 
Michael C said:
What I'm interested to know is why do some usb devices have them while
others survive quite well without.
Click to expand...

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

https://gullfoss2.fcc.gov/prod/oet/cf/eas/reports/GenericSearch.cfm

and access the reports which were filed when the device was
certificated.

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.
 
Why do some of my USB leads (for example those which came with my
dictation machine) have a largish cylindrical plastic "blob" on them?

The blob is about an inch long and half an inch is diameter.
Click to expand...


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

The Chinese don't care. The US companies will use them as needed to comply
with FCC rules.
 
Michael said:
What I'm interested to know is why do some usb devices have them while
others survive quite well without.
Click to expand...

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
 
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.
Click to expand...

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?
 
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?
Click to expand...


Do you see anywhere where I wrote "stupid", you stupid motherfucker?

For the device that it is, the answer was fine. For the lay person
that was given the answer, the answer was fine. Ther was no need for
a detailed response about ****ing ferrite EMI traps.

Grow the **** up, you are a piss poor judge of anything social, and
apparently not too good on the technical side either.
 
Do you see anywhere where I wrote "stupid", you stupid motherfucker?

For the device that it is, the answer was fine. For the lay person
that was given the answer, the answer was fine. Ther was no need for
a detailed response about ****ing ferrite EMI traps.

Grow the **** up, you are a piss poor judge of anything social, and
apparently not too good on the technical side either.
Click to expand...

You are SO easily rattled. Funny. You thought that would hurt? I wouldn't
have called out your too-little too-late cleverness if I was worried about
that. While we have our silly little side-show, I imaging the OP is busy
with the sensible answers already given. You might igmore them, but anyone
else can see that the subject is already answered. People often put their
bit in as extra but they usually try to add something instead of trying to
make the OP look like a plonker for asking a simple and sensible question.
 
You are SO easily rattled. Funny. You thought that would hurt? I wouldn't
have called out your too-little too-late cleverness if I was worried about
that. While we have our silly little side-show, I imaging the OP is busy
with the sensible answers already given. You might igmore them, but anyone
else can see that the subject is already answered. People often put their
bit in as extra but they usually try to add something instead of trying to
make the OP look like a plonker for asking a simple and sensible question.
Click to expand...


This is an engineering group, dipshit. That question belongs in a
basic electronics group, at best.

I read the OP and didn't read, nor did I need to read ANY of the
other responses to make my own.

So again, jackass... **** off, and NO I am not angry, I am ****ing
laughing at your retarded ass.
 
MassiveProng said:
.... snip ...

This is an engineering group, dipshit. That question belongs in
a basic electronics group, at best.

I read the OP and didn't read, nor did I need to read ANY of the
other responses to make my own.

So again, jackass... **** off, and NO I am not angry, I am
****ing laughing at your retarded ass.
Click to expand...

PLONK for stupid obscenity.

--
<http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfocus.com/columnists/423>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews
 
MassiveProng said:
This is an engineering group, dipshit. That question belongs in a
basic electronics group, at best.
Click to expand...

This is cross posted to 3 groups all of which are applicable. Besides, I
don't see what the problem is coming into an ee group and asking a question
like this. I mean, what better place to ask it? One would assume (but be
mistaken in your case) you'd get intelligent replies there.
I read the OP and didn't read, nor did I need to read ANY of the
other responses to make my own.
Click to expand...

Oh yeah :-)
So again, jackass... **** off, and NO I am not angry, I am ****ing
laughing at your retarded ass.
Click to expand...

If you're trying to save face you are just making yourself look more and
more stupid. What is that saying about it being better to remain silent than
open your mouth and reveal you are a fool, well you've well and truly
revealed what a fool you are. :-)

Michael
 
Well you sound pretty ticked off to me.

It also sounds like your default reply mode is just to hurl insults,
whether someone asks a question you don't like,
disagrees with you,
or (horrors) criticizes you, even constructively.
The tone of your posts is pretty consistent.

Note that very few other posters reply in such a manner,
on this or pretty much any other newsgroup,
and some of them even know more than you do about the topic.
Perhaps when you grow up you won't act like this.
------------
 
This is cross posted to 3 groups all of which are applicable.
Click to expand...

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

Besides, I
don't see what the problem is coming into an ee group and asking a question
like this. I mean, what better place to ask it? One would assume (but be
mistaken in your case) you'd get intelligent replies there.
Click to expand...

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.
 
Why do some of my USB leads (for example those which came with my
dictation machine) have a largish cylindrical plastic "blob" on them?

The blob is about an inch long and half an inch is diameter.
Click to expand...
Watch who you're calling a blob!
When the Artic melts, watch out. You know what they put up there.
 
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