Hi Chas,
Wrapping the cable around the ferrite for two passes works below 200 MHz
but above that it can reduce the effectiveness of the ferrite because of
capacitance between the turns. Also if you wrap the Ethernet cable
tightly around the ferrite, you may slightly unbalance the twisted pairs
and offset the help of the ferrite. But if you leave a large loop of
cable, the phase shift around the loop will hurt effectiveness at higher
frequencies. Seems like nothing is ever simple.
I am writing my Ethernet balance Technical Tidbit for August now. It
should be out by the weekend.
Doug
On 7/26/11 12:32 PM, Grasso, Charles wrote:
> Hello Doug,
>
>
> I was interested in your comments regarding the degrading of Ethernet cables
and poor EMI performance as a result.
> Typically we run Enet cables into a test chamber to provide traffic for the
ports. We (of course) add ferrites
> external to the chamber to mitigate the effects of ambients in the chamber.
We also run one wrap around the ferrite
> to increase the effectiveness of the ferrite. What is your opinion of this
practice?
>
> Chas
> ******************************************
> .
>
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