NFC/RFID devices -- How do they transmit data?

[H Smith 94]

Hi all,

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 realized 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?

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

 

[nsaspook]

A passive device modulates the carrier field by adjusting the effective load resistance on the secondary (the passive device pickup coil). A simple example would be a 1:1 transformer (power side–channel) circuit. A 1.0 mA current draw on the secondary means a 1.0 mA draw on the primary, if we drop the current draw on the secondary to 0.5 mA we now have a 0.5 mA current on the primary. If we quickly alternate the load currents with 1.0mA as a binary 1 and 0.5mA as binary 0 we have a means to transmit data back to the initiator device.

A active device will have it's own antenna to generate a field to send back to the initiator device.