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RFID Tags

RFID tags come in two distinctly different types, active and passive. Each of these types comes in a variety of frequencies (see RFID Frequency for more information of allowable frequencies). Both types of tags have variations and there are applications for both.

Passive RFID (pRFID) uses a mechanism called backscatter to communicate from the tag to the reader. The tag is built from a “chip” (an integrated circuit) and an antenna. The chip derives the power it needs to work from the signal transmitted by the reader.

ISO/IEC 19762-3 (Information technology AIDC techniques – Harmonized vocabulary, Part 3 – Radio-Frequency Identification (RFID)) defines backscatter as “process whereby a transponder responds to a reader/interrogation signal or field by modulating and re-radiating or transmitting the response signal at the same carrier frequency“. What this means is that that tag does not have a transmitter on board. It relies on the ability to modulate the carrier wave of the reader/ interrogator to respond to the reader.

The advantage of a passive tag is that it does not have any power supply (battery) on the tag and so is very inexpensive to make. The tag is made up of just a chip and an antenna.

The disadvantage of a passive tag is exactly the same thing, it does not have a power supply. This lack of power means that the tag needs to harvest enough energy from the radio signal to power up the chip. This effectively limits the range that the tag can operate over (depending on frequency this is typically in the 50 cm to 5m range). The backscattered signal can be detected over a much greater distance than the longest distance from the reader to the tag that provides enough power to make the chip run.

The lack of power has led to the development of battery assisted passive tags (BAP). These tags are the same tags as a passive tag described above with the addition of a battery to supply power to make the chip run. The tags still backscatter the signal but because the power for the chip comes from the battery, that tag has a much greater range (some vendors are quoting 20 – 30m for BAP tags).

The second type of tag is an active tag.  This tag also has a power supply (typically a battery), but the tag also has a transmitter on board.  It does not rely on backscatter techniques to send a signal back to the reader. This ability to actually transmit a signal means that the range of an active tag can easily reach 100 – 150m.

This big range advantage of active tags is offset by the need for the batter and the more complex electronics needed. Tags are not as small as passive tags and have a battery life of 3 – 7 years depending on the manufacturer.