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USB 2.0 and FCC/CE EMC compliance

Discussion in 'Electronic Design' started by galapogos, Jul 2, 2007.

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  1. galapogos

    galapogos Guest

    Hi guys,

    I'm developing a USB 2.0 high speed device that is currently very much
    functional. However, I'm having some problems with radiated emissions
    test with both FCC and CE standards. Based on the sweep from
    30-1000MHz, I'm pretty convinced it's a USB 2.0 problem, because I see
    distinct spikes at 240, 480, 720, 960MHz, which are probably harmonics
    of the 480MHz USB 2.0 sweep. These spikes occur on 2 different PCs
    that I use, and they disappear when I use another laptop that only has
    a USB 1.1 port.

    I'm wondering if anyone has had experience designing USB 2.0 devices
    for FCC/CE compliance? I understand that many factors could cause
    these spikes, ranging from the power source, actual test PC used,
    cable length/shielding/gauge, cable layout, etc...I'm wondering which
    ones are the ones that cause the biggest difference. Booking the test
    lab is kinda expensive and I have no equipment to perform debugging so
    I hope I can do the most I can before hand to solve this problem
    before showing up at the lab.

    Thanks in advance!
     
  2. Regarding cables etc. ask the range engineer how this is typically
    done. I have not done FCC for USB but for other devices. How bad are
    you failing? When you quasi-peaked were you still over? Is your
    device shielded? In addition to the things you listed above, it could
    actually be your device which is failing FCC Class B...when you were
    connected to a usb 1.1 port the 480MHz clock on your device was
    probably not operating. When you connected to the two different PCs
    it was operating...just pointing this out.

    If your device is not shielded go to the grocery store and buy some
    disposable baking pans cut out appropriate holes for USB, power, etc.
    At the range with the baking pan in place, does your emissions go
    down? If your emissions goes down it is probably an issue with your
    design. You will need to look at PCB layout etc.

    regards,
    Kadir "Solid Gold" Suleyman
     
  3. galapogos

    galapogos Guest

    Thanks. My device is enclosed in a partially metallic(steel)
    enclosure. An older design had a fully metallic enclosure, and the
    spikes were slightly less but still there. The layout/design for the
    older design is different though, so it's not an apples to apples
    comparison. I thought about wrapping the device up with aluminum foil
    as well, but seeing that the older design was already fully shielded,
    would that help?

    Unfortunately for us, the testing engineer isn't very interested in
    helping us pass. He's just doing his tests and telling us that we're
    failing without providing us with much help, and this is the first
    time I'm doing FCC/CE testing, so I'm kinda lost as to what I should
    be doing as well. I'm hoping it's a simple cable issue, but I'm just
    not sure.
     
  4. Hmm.. Partially metalized is pretty good for a consumer device. You
    never mentioned how bad you are failing with the spikes. It's
    impossible to say without looking at your layout, and having someone
    who is knowledgeable of USB testing advise you of the correct
    procedure. But...

    Do you have an EMC analyzer? If you can at least rent one, and a
    close-field probe (or make one using coax and an improvised antenna),
    you should be able to probe around and take relative readings and
    isolate out where the radiation is coming out. Check the data sheet
    and make sure you don't have any unnecessary clocks turned on. Try
    both shielded and unshielded cables.

    Eliott labs (Silicon Valley area) is an excellent EMC lab and you can
    ask their EMC engineers questions. First thing I would do, tho is
    check your layout.
     
  5. Eeyore

    Eeyore Guest

    Did you do any pre-compliance testing ?

    What design principles did you adhere to in order to minimise your emissions ?

    Graham
     
  6. galapogos

    galapogos Guest

    The spikes are pretty bad. For FCC, here's how much it's failing by
    240MHz - 20.2dB
    480MHz - 17.9dB
    720MHz - 22dB
    960MHz - 16.8dB

    For CE, here's how much it's failing by
    240MHz - 14.2-25dB
    480MHz - 8.7-11.5dB
    720MHz - 6.2-18.3dB
    960MHz - 2.4-5.4dB

    Theses are over a few different PCs.

    I might have access to a spectrum analyzer, but I'm not sure if I have
    antennas or near field probes available. I'll check though. I'm not in
    the USA so I don't think I'll be able to use their services.
     
  7. galapogos

    galapogos Guest

    No pre-compliance testing. We followed mainly reference designs, and
    for the USB/IDE portions(this is a USB storage device), we adhered to
    USB and IDE design guidelines for trace length/PCB stackup/etc. This
    is mainly for SI issues. Emissions was not specifically designed for.
     
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