Method of Operation
The card and the reader unit communicate with each other through radio frequency fields of either 125 kHz, or 13.56 MHz for the newer cards, by a process called resonant energy transfer. Passive cards have three components, sealed inside the plastic: an antenna consisting of a coil of wire, a capacitor, and an integrated circuit (IC), which contains the user's ID number or other data. The reader unit has its own antenna, which continuously transmits a short range radio frequency field.
When the card is placed within range of the reader, the antenna coil and capacitor, which form a tuned circuit, absorb and store energy from the field, resonating like an electrical version of a tuning fork. This energy is rectified to direct current which powers the integrated circuit. The chip sends its ID number or other data to the antenna coil, which transmits it by radio frequency signals back to the reader unit. The reader checks whether the ID number from the card is correct, and then performs whatever function it has been programmed to do. Since all the energy to power the card comes from the reader unit, passive cards must be close to the reader to function, and so have only a limited range.
An active card contains a flat lithium cell in addition to the above components, to power it. The integrated circuit contains a receiver which uses the battery's power to amplify the signal from the reader unit so it is stronger, so it can detect the reader at a greater distance away. The battery also powers a transmitter circuit in the chip which transmits a stronger return signal to cover the greater distance.
Read more about this topic: Proximity Card
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