Editing Proximity card (section)

== Method of operation ==
The card and the reader unit communicate with each other through 125&nbsp;kHz radio frequency fields (13.56&nbsp;MHz for the [[contactless smartcard]] cards) by a process called [[resonant inductive coupling|resonant energy transfer]].<ref name="Reid" /><ref name="Norman" />  Passive cards have three components which are 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 in specific formats and no other data.  The reader 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, [[resonance|resonating]] at the frequency emitted by the reader. This energy is [[Rectifier|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 for that ID number. All the energy to power the card comes from the reader unit, so passive cards must be close to a reader to transmit their data.

An active card contains a flat [[lithium cell]] in addition to the above components to power it. The integrated circuit contains a [[Receiver (radio)|receiver]] which uses the battery's power to [[amplifier|amplify]] the signal from the reader unit so it is stronger, allowing the card to detect the reader at a greater distance. The battery also powers a [[transmitter]] circuit in the chip which transmits a stronger return signal to cover the greater distance.