Passive Tags

Passive tags (Figure 10-1) are the most widely used in RFID applications. Passive RFID tags have no internal power supply because they are powered by RFID readers using techniques such as load modulation, reflection, or backscatter. The incoming RF signal powers the integrated circuit in the tag and sends a response after the signal is modulated and sometimes amplified. The antenna for a passive RFID chip must be designed to both receive and transmit data. Since the entire analog and digital circuit inside the tag must be powered by the RF signal emitted by the reader, the amount of energy available for the return transmission from tag to reader is extremely limited. Transmission distance, therefore, will continue to be an issue with passive tags.

The FCC imposes power standards to RF equipment that also limits the range of the tags to less than twenty feet. Most applications, such as proximity badges, will only work within a few inches of the reader.

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