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

  1. Jul 14, 2015 #1

    H Smith 94

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    Gold Member

    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 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?

    As a physicist I have very little prior exposure to electrical engineering or system design!
  2. jcsd
  3. Jul 14, 2015 #2


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    Science Advisor

    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.
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