[SI-LIST] Re: EMI--bare board vs. case
- From: "Vishram Pandit" <vishrampandit@xxxxxxxxxxx>
- To: yost@xxxxxxxxxxxxxxxxxxx, Si-list@xxxxxxxxxxxxx
- Date: Tue, 22 Apr 2003 02:53:52 +0000
Boris,
Overall there are 3 factors influencing radition:
1] Originating sources
2] Coupling mechanisms
3] Final radiating elements.
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.
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.
Hope this helps...
Thanks,
Vishram
>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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