Passive Tag

Passive Tag

A passive RFID tag contains no internal battery. It harvests all operating energy from the interrogator's RF field, uses that energy to power its Tag IC, and communicates back to the reader by modulating the reflected signal -- a technique called backscatter.

How Passive Tags Work

When the reader's RF carrier wave reaches the tag antenna, an AC voltage is induced. The tag IC's charge pump rectifies this AC into DC, and a voltage regulator stabilises it to power the digital logic. Once the IC has sufficient charge (above the tag sensitivity threshold, typically -18 to -24 dBm), it begins processing the reader's commands and modulating its antenna impedance to encode data onto the reflected wave.

This impedance modulation creates amplitude or phase shifts in the backscattered signal that the reader decodes. The modulation scheme used for tag-to-reader communication is either FM0 or Miller encoding, selected by the reader based on the RF environment.

Performance Characteristics

Advantages and Limitations

Passive tags excel in cost-sensitive, high-volume applications like item-level tagging in retail apparel, where billions of tags are consumed annually. Their thin, flexible form factor allows embedding in smart labels and product packaging. The absence of a battery means indefinite shelf life and no maintenance.

The primary limitation is range: passive UHF tags rarely exceed 12 m in ideal conditions and degrade significantly near liquids and metals (detuning). For assets requiring longer range or environmental sensing, active tags or semi-passive tags are preferred.

See also: Active Tag | Semi-Passive Tag | Tags