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Using chip ferrite beads

Discussion in 'Electronic Design' started by galapogos, Jul 18, 2007.

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  1. galapogos

    galapogos Guest


    I have a design that I've found to be radiating EMI with a spectrum
    analyzer and a few near field probes. My design currently doesn't have
    any EMI filtering. I'm thinking of using ferrite beads to reduce EMI.
    Other than the power and ground lines, I'm wondering where else would
    they help? Here are a couple of places I thought may help

    - My design has a membrane keypad that connects via an FPC connector.
    The keypad acts like an antenna, and radiation is vastly reduced when
    I place it on a metal surface. Since right now I'm unable to change
    the mechanical design/material of the casing, I'm thinking of of
    putting ferrite beads on all of the keypad signals. These are very low
    frequency signals so I think should be fine?

    - My design has a daughterboard that is connected with a 2x22 IDE type
    connector that transfers IDE signals. I also find that this connector
    leaks quite badly. However, since IDE signals are pretty high speed,
    I'm wondering if using regular ferrite beads will cause any signal

    Btw board boards have separate power and ground planes. The ground
    plane of the daughter board is connected via the ground pins of the
    IDE signal, and the daughter board actually helps to shield radiation
    coming from the main board. I've tested this with the probes with the
    daughter board over the main board(and not connected)...radiation is a
    lot less than if the main board was naked.

    While I didn't design the board with EMC in mind, many of the SI
    design guides were followed, which help EMC too, from what I've read.
    These include a separate power/gnd plane, correct impedance matching
    with trace width/length/stackup configurations, trace length matching
    for the high speed signals, trace parallelism for the differential
    signals, decoupling caps as near to the chip as possible, etc. These
    are all according to the chip/IDE/USB design guidelines...wondering
    what else I can do to reduce the EMI radiation? I've tried pasting
    aluminum tape(helps), and EMI absorber sheets(doesn't help at all).

  2. Since any current heading towards the keypad should be
    canceled by a current returning from the keypad (except for
    the generation of RFI), you might start out with a single
    ferrite bead around the FPC cable.

    Here is a picture of one of these: photos/28R0898-200.jpg

    Not if it surrounds the entire cable and no other grounds or
    signal paths exist between these two modules.
  3. Robert Baer

    Robert Baer Guest

    Without changing the mechanical design of the keyboard, one could
    have the inside of the case coated with Acheson nickel-iron equivalent
    of Aquadag.
    This gives the shielding you saw, and absorbs RFI as well.
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