Inductive Coupling

Inductive Coupling

Inductive couplingInductive couplingMagnetic field induction enabling power transfer and bidirectional communicationView full → is the electromagnetic mechanism by which NFC devices communicate and transfer energy. The NFC reader's coil antenna generates an alternating magnetic field at 13.56 MHz, which induces a corresponding current in the NFC tag's coil antenna. This single physical phenomenon simultaneously accomplishes two tasks: powering the passive tagpassive tagBatteryless tag powered by reader's electromagnetic fieldView full → and enabling bidirectional data transfer.

The Transformer Analogy

Inductive coupling in NFC operates on the same principle as an air-core transformer. The reader's antenna coil acts as the primary winding, and the tag's antenna coil acts as the secondary winding. The coupling coefficient (k) between the two coils determines how efficiently energy transfers from reader to tag:

• k = 1.0: Perfect coupling (theoretical maximum, never achieved in practice).
• k = 0.01-0.1: Typical NFC operating range. Even this modest coupling is sufficient because NFC tags require very little power (microwatts to low milliwatts).
• k approaches 0: Tag is too far from the reader or misaligned. Communication fails.

The coupling coefficient depends on the distance between coils, their relative sizes, alignment angle, and any intervening materials (particularly metals that create eddy currents).

Data Transfer via Load Modulation

Once the tag is powered, data flows in both directions through the coupled field:

• Reader to tag (downlink): The reader modulates the carrier amplitude (ASK modulationASK modulationSignal amplitude variation encodingencodingData writing to NFC tags during manufacturing productionView full → data on 13.56 MHz carrierView full →) to encode commands. NFC-A uses 100% ASK with Modified Miller codingModified Miller codingBit encoding for NFC-A reader-to-tag communicationView full →; NFC-B uses 10% ASK with NRZ-L coding.
• Tag to reader (uplink): The tag cannot generate its own field. Instead, it switches a load resistance across its antenna coil, varying the impedance the reader sees reflected through the coupled field. This load modulationload modulationPassive tag response technique varying load impedanceView full → creates subcarrier sidebands at 13.56 MHz plus or minus 847.5 kHz (or 423.75 kHz) that the reader demodulates to recover the tag's response.

Design Implications

Antenna designAntenna designEngineering NFC antennaNFC antennaCoil antenna creating electromagnetic field for NFC communicationView full → geometry for performance requirementsView full → is critical for reliable inductive coupling. The NFC antenna must be tuned to resonate at 13.56 MHz for maximum energy transfer. Factors like trace width, number of turns, substrate material, and proximity to metal surfaces all affect the tuning. On-metal tags use ferrite shielding layers to prevent eddy current losses in the metal substrate from detuning the antenna and collapsing the coupling coefficient.