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Feature Article
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Issue #211 February 2008
RFID Payment Terminal
by Carlos Cossio
Start | System Overview | Energy Transmission | 100% Amplitude Modulation | Load Modulation | MIFARE Card | MIFARE and ISO 14443 | Pay as You Go | Hardware Design | Antenna Design Rules | Antenna Size | Directly Matched Antenna | Firmware Design | Catch the Wave! | Sources & PDF
ENERGY TRANSMISSION
Energy transmission between the reader antenna and the passive contactless card is based on the transformer principle. On the reader side, an antenna coil and a card coil implemented in the contactless card are required. Figure 1 shows the basic principle and the equivalent electronic circuitry. The antennas and energy transmission are depicted on the left-hand side of Figure 1.
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| Figure 1—The transformer model is used to transfer energy and data wirelessly between a reader and a contactless card using inductive coupling. |
The current “I” in the reader antenna coil generates a magnetic flux. Parts of this flux flow through the card coil and induce a voltage in the card coil. The voltage is rectified and the card IC is activated when the operating voltage is reached. The induced voltage varies within the distance between the reader antenna and the contactless card. Due to that voltage variation, the achievable operating distance is limited by the transferred power. The right-hand side of Figure 1 shows the equivalent electrical circuitry, the transformer model.
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