[SI-LIST] Re: EMI--bare board vs. case

Vishram, Boris,

Just want to add a small comment - all of Vishram's considerations are 
valid if the case is metal.

In a plastic case - PCB and attached cables are the main source and antenna 
for emissions and, because of this, everything needs to be done to reduce 
an EMI source of emissions.

In same cases, even a metal enclosure could not help. For example, when an 
Ethernet 10/100/1000 BaseT Cu port had been designed  poorly.

Best Regards, Val

          At 02:53 22/04/2003 +0000, Vishram Pandit wrote:
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>Boris,
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>Overall there are 3 factors influencing radition:
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>1] Originating sources
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>2] Coupling mechanisms
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>3] Final radiating elements.
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>It is required to reduce the source level EMI as far as possible. Power and
>ground noise is major contributor for that. Enclosure provides certain
>shielding effectiveness. A bare board is connected to the enclosure on
>various points by screws or any other mechanism. This is sometimes very
>useful for the EMI. In such a case, the improvement in the EMI is not only
>due to the enclosure, but also due to the ground path. Therefore, (M dB
>generated) - (N dB attenuated) = (P dB picked up by the FCC)  may not work.
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>Also, final radiating elements are cables. So it depends on if you are dong
>the enclosure experiments with or without the cables. For some frequencies,
>the radiation may be the same with and without the enclosure, when cables
>areconnected. In this case also, (M dB generated) - (N dB attenuated) = (P
>dBpicked up by the FCC)  may not work.
>
>Enclosures are designed for say half wavelength of the 10th harmonic of the
>the max. freq. on the board. Still we can see the intermediate frequencies.
>This is due to multiple reasons: inadequate grounding, radiation from the
>edge of the board, cabling issue, etc.
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>Hope this helps...
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>Thanks,
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>Vishram
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> >From: "Boris Yost" >Reply-To: yost@xxxxxxxxxxxxxxxxxxx >To:
>"Si-list@freelists. org" >Subject: [SI-LIST] EMI--bare board vs. case >Date:
>Mon, 21 Apr 2003 20:43:53 -0400 >>>Dear List: >>Is there any expectation for
>EMI emissions of a bare board vs. an assembled >system? Does anybody ever
>test a bare board for EMI...with the idea that a >case is expected to
>attenuate N dBuV and then it will be low enough to pass? >Or have a standard
>for noise on power/ground planes somehow correllated with >emissions? Or,
>more pointedly... >>Is it as simple as: >(M dB generated) >- (N dB
>attenuated) >__________________ >(P dB picked up by the FCC) ? >>Boris
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Val Mandrusov
Sr. Compliance Engineer

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