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I am designing an NFC device but am still a little unsure about the principles behind NFC/RFID. From what I understand:

  1. A primary coil constantly emits a 'carrier frequency' magnetic field.

  2. This field induces an e.m.f. on a passive secondary coil which is in a secondary circuit.

  3. The secondary circuit comprises a system that converts data into a modulation signal, which is realised by a modulating impedance in the second circuit.

  4. This impedance causes something in the primary circuit to change, thus receiving data.

It is step 4. that I am most confused by. Sources I've read simply say the load impedance is "felt" by the primary coil, whatever that means.

Also, what is the function of having a separate transmit and receive antenna on NFC transceivers?

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Partial answer:

When a resonant coil is "unloaded " the current and voltage will be in quadrature (90 degrees out of phase). If you bring a "load" into the vicinity of the coil, this will bring the current more in phase with the voltage (it does work on the load). So if you have a passive secondary coil that can only change how "visible" it is (by changing its impedance) then the primary could can feel this because it sees a phase shift.

The simplest way to change impedance is with a tiny switch that opens and closes the loop, obviously.

I can't answer the part of the question about separate antennas.

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