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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Common-Mode Rejection by Coils/Transformers and Other Techniques
Besides the most basic requirement of a twisted pair, what special techniques were we discussing previously ? Let’s list some of them here.
1.  We start by revealing the simplest technique to separate signal from noise, applicable to both analog (telephony) and digital (Ethernet). Visualize the following situation: If both ends of a magnetic coil, such as in the loudspeaker of a telephone (the first schematic in Fig. 1.5), or one of the windings of say, a data transformer (the last schematic in Fig. 1.5), are raised or lowered in unison by exactly the same amount (and that by definition is common-mode), no corresponding current will be produced in the coil. Why? Because current ...

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