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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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The Center-Tapped (Hybrid) Transformer and the Phantom Circuit
In the top schematic of Fig. 1.5, we see the signal from the microphone being transmitted down a twisted pair to a loudspeaker inside a remotely located telephone. That ensures Person A can talk to Person B. But what about reverse communication? We need Person B to talk to Person A too, and simultaneously. We could use a second twisted pair for that. See the uppermost schematic of Fig. 1.8. That works, but it is neither smart nor cheap. Can we save one twisted pair? The historical answer to that was the hybrid transformer. It is presented in a very simplified form in the second schematic in Fig. 1.8 (shown in more detail in Chap. 13). We are not going to do any math here, but we ...

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