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

Discussion in 'Electronic Components' started by Jon D, Feb 7, 2007.

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  1. Jon D

    Jon D Guest

    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.
  2. Palindrome

    Palindrome Guest

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

    Don Bruder Guest

    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.
  4. Guest

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

    Rod Speed Guest

    Nope, its to stop the cable radiating.
  6. Michael C

    Michael C Guest

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

    Dave Platt Guest

    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

    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

    and access the reports which were filed when the device was

    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.
  8. MassiveProng

    MassiveProng Guest

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

    Eeyore Guest

    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.

  11. Eeyore

    Eeyore Guest

    The Chinese now have to meet IEC regs actually.

  12. 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
  13. MassiveProng

    MassiveProng Guest

    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 fucking 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.
  14. 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.
  15. MassiveProng

    MassiveProng Guest

    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 fucking
    laughing at your retarded ass.
  16. CBFalconer

    CBFalconer Guest

    PLONK for stupid obscenity.


    "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
  17. Michael C

    Michael C Guest

    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.
    Oh yeah :)
    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. :)

  18. Wow

    Wow Guest

    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.
  19. kony

    kony Guest

    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.
  20. MassiveProng

    MassiveProng Guest

    Do NOT top post in Usenet, you fucking interloping, forum invading
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