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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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EMI Filtering and Common-Mode Filters with PoE
As discussed in Chap. 1, there are two EMI-related concerns that make twisted pairs so attractive: emissions and susceptibility. Noise is picked up in common-mode manner and it is important to not let it affect the signal, which is transmitted in a differential-mode manner. Remember that in Chap. 2 (see Fig. 2.10), we provided the rationale behind adding two small ferrite beads in each line, to prevent the center-taps of the send and receive transformers from getting connected together from the viewpoint of the high-frequency noise signals. In addition to that, common-mode filters are generally recommended in series with the PoE and data lines too.
In many cases it is OK to just put a small common-mode ...

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