Low and High Frequency EMI shield

Discussion in 'The Projects Forum' started by dude521, Jan 27, 2009.

  1. dude521

    Thread Starter Active Member

    Nov 1, 2008
    37
    0
    Hi guys,

    I have a device which needs to take very sensitive measurements and I want to block all the electromagnetic interference from AC power lines and the transformer I have in the device (60Hz), the DC field from the power supply (~0 Hz), and the field from an oscillator (~700 Hz).

    I am looking at doing two things, the first is to build a faraday cage to block the high frequency fields (60 and 700 Hz). This I will do by taking copper or aluminum sheets to build a box around the board that does the measurement. The second thing I want to do is surround this box with Mu-metal so that the static fields go around the box.

    From what I understand, the static fields will bend around the Mu-metal and not pass through, while the changing fields will induce an opposite field inside the copper box to cancel out the field.

    Any pointers from people who have done this before would be great. I think I should have the boards inside the faraday cage (copper) and then have the entire faraday cage inside the Mu-metal cage. Does that sound right or will the Mu-metal also act like a faraday cage?


    Thanks for the help.
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287

    The high permeability of the Mu metal works best on low frequencies...such as AC magnetic fields. The copper works best at R.F. frequencies because of Faraday shieldiing. Somewhere in my deep dank basement I have a shielding design book with lots of numbers and graphs and nomograms. However, you're on the right track.

    Mercy...what does Mu metal cost these days?

    :eek:

    Eric
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Given costs, you might want to make the Faraday cage first and then determine if you need the additional shielding. Hope you have a really good ground to connect to the Faraday cage. The one in your house/lab electrical service won't be enough - think dedicated service.
     
  4. dude521

    Thread Starter Active Member

    Nov 1, 2008
    37
    0
    Thanks guys, I'll make the faraday cage first with copper then decide on the Mu-Metal.

    I was wondering, if I do use both, should the Mu-metal box be inside the faraday cage or the other way around?

    Also does anyone have any links to construction information. I can't really find too many site that talk about making one of these.

    Thanks.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
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