I am designing an NFC device but am still a little unsure about the principles behind NFC/RFID. From what I understand:
A primary coil constantly emits a 'carrier frequency' magnetic field.
This field induces an e.m.f. on a passive secondary coil which is in a secondary circuit.
The secondary circuit comprises a system that converts data into a modulation signal, which is realised by a modulating impedance in the second circuit.
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?