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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?

As a physicist I have very little prior exposure to electrical engineering or system design!

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The transmitter to receiver coupling is similar to how a transformer works. for a ideal transformer, whatever impedance is connected to the secondary appears as the impedance of the primary divided by the square of the turns ratio.

For example, imagine a 7 V AC source and a 1:2 stepup transformer. If you connect a 3 Ω resistor directly to the 7 V, it will draw 2.3 A. If you instead put the transformer between the source and the resistor, the resistor now sees 14 V applied to it. It will draw 4.7 a from the secondary, which means 9.3 A must be going into the primary, and the resistance seen by the 7 V source is 3/4 Ω.

Now to get back to your question, surely you can see that a circuit could detect this 750 mΩ on it's 7 V supply if designed to do so.

The coupling between transmitter and receiver is not like a ideal transformer, but changes in impedance presented to the secondary (the coil in the receiver) still cause apparent changes in the impedance of the primary (the coil in the transmitter). It's a small matter of engineering to properly detect the pulses of different impedance the receiver is putting on its receiving coil, turn them into 1s and 0s, and decode this digital stream to get the data payload encoded onto it by the receiver.

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