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A reader transmits both data and energy to the transponder via inductive coupling. To operate the contact-free transponder, the reader first generates a high-frequency magnetic field in its antenna. This magnetic field is around 13.56 MHz. If the transponder is then held near to the read antenna, the reader's field induces a potential difference in the coil of the transponder. This is rectified and serves as a voltage supply for the transponder. The transponder therefore needs no voltage supply of its own from a battery.
A capacitor is connected in parallel with the inductivity of the transponder coil. This creates a parallel resonant circuit, the resonance frequency of which corresponds to the transmitter frequency. At 13.56 MHz, the input capacitance of the transponder chip is sufficient for this. To transmit data from the chip to the read antenna (known as the uplink), this property of the transformer-type coupling between read antenna and chip card coil is used. By switching an additional load resistance in the transponder chip on and off, a clock frequency is generated via which data can be sent to the reader. This process is known in the technical lexicon as load modulation.
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