Loading up the PCI slots puts more capacitance on the PCI bus, slowing the
logic transistions and thus reducing emissions. Quite likely loading up the
ISA slots will do the same thing, though emissions could go up.
To reduce keyboard emissions, try wrapping the cable multiple times through
a ferrite torroid. Inductance is proportional to the square of the number
of turns, so just a few turns can be a lot more effective than a single
pass through a bead. If the problem emissions are in a narrow range of
frequencies, you can adjust the number to turns to tune the point of
parallel resonance between the interwinding capacitance and the inductance
to make a large series impedance at the frequency of interest.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA, USA
Derek N. Walton wrote:
> Hi All,
>
> recently I posted about my tribulations with a noisy motherboard.
>
> I was testing with just the bare minimum of components, which
> included cards in the slots. For grins I loaded up the PCI slots
> with cards, and the system got quieter. Note, there were no cables,
> just the cards plugged in. And yes, the back panel holes were
> covered when the cards were not in. :-)
>
> This is an older MB, with ISA slots too. I have no cards for these.
> Could they be ringing, and causing the few spikes I have left? If so
> are there dummy loads I could plug in the slots to damp the ringing?
> The same board has multiple uses......
>
> Also noted that adding the keyboard dramatically adds to the
> emissions, the mouse not so much. The Fe bead on the cable doesn't
> help much...
>
> Thoughts anyone?
>
> Thanks,
>
> Derek N. Walton
> Owner L F Research EMC Design and Test Facility
> Poplar Grove,
> Illinois, USA
> www.lfresearch.com
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