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Shielding, connectors, filtering!

Discussion in 'Electronic Design' started by Brad, Apr 5, 2008.

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  1. Brad

    Brad Guest

    Hello everyone! Thanks for your comments on my questions below!

    1.) The best case for shielding is a fully enclosed metal box. Would
    this be equivalent to a shield on one side of the PCB with a ground
    plane on the opposite side of the PCB?

    2.) What is the best approach when you have a break in your metal
    shielding for connectors? Should I bring the shielding all the way
    around the connector? If I do this how should I route the lines?
    Under the shielding with a via? Or through the shielding? Or just
    keep the gap open for the connector?

    3.) We had big ferrite clamps that we put on the signal cables and
    power supply. I would not like to have these large clamps on the
    cable and fix things inside of the device. The largest bandwidth for
    our signals is 500 Hz. We were seeing problems above 30 MHz for our
    emissions testing with the cables. Would it make sense to filter the
    signals going out on the cables? Or is there a bead that is small that
    would do the trick inside of the device?

    Thanks in advance for your comments!

    -Brad
     
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  3. Guest

    For any faradays cage the holes should be less than half the wavelength.
    (in microoven it's 1/64)
    Any conductor in/out will work as an rf "tunnel".
    Shielding all the way I presume..
    Maybe some RLC stuff?
     
  4. Guest

    The "big ferrite clamps" are a quick and dirty way of making a "balun"
    - otherwise a balanced transformer - which is used to block common
    mode currents that would other wise circulate around the (common)
    ground connection.

    The clamps are big only because they have to fit around the cables -
    the inductances involved aren't all that high - of the order of a
    microhenry - and it is easy enough to to create comparable inductances
    (and mutual inductances) on a printed circuit board with a suitable
    two-hole ferrite bead. There are comparable surface mount ferrite
    chips - Farnell lists a bunch of TDK and Murata SMD "suppression
    chokes" while similar o parts from other manufacturers are listed in
    the same section of their catalogue - section 8, EMC filters and
    suppressors, as "common mode chokes" and "data line chokes".
     
  5. Guest

    You need the help and advice of a real EMC Engineer. Band-aid fixes
    don't work very well and that is what you are exploring. Pay some $$
    and buy a ciouple of good EMC books. Tim WIlliams' book is good, also
    anything by Clayton Paul or Mark Montrose.

    Bob Hofmann
     
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