# Technology Brief 13: RFID Systems

In 1973, two separate patents were issued in the United States for Radio Frequency Identification (RFID) concepts. The first, granted to Mario Cardullo, was for an active RFID tag with rewritable memory. An active tag has a power source (such as a battery) of its own, whereas a passive RFID tag does not. The second patent was granted to Charles Walton, who proposed the use of a passive tag for keyless entry (unlocking a door without a key). Shortly thereafter, a passive RFID tag was developed for tracking cattle (Fig. TF13-1), and the technology rapidly expanded into many commercial enterprises—from tracking vehicles and consumer products to supply chain management and automobile anti-theft systems.

# Figure TF13-1 Passive RFID tags were developed in the 1970s for tracking cows.

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# RFID System Overview

In an RFID system, communication occurs between a reader—which actually is a transceiver—and a tag (Fig. TF13-2). When interrogated by the reader, a tag responds with information about its identity, as well as other relevant information depending on the specific application.

# Figure TF13-2 How an RFID system works is illustrated through this EZ-Pass example. The image of the UHF RFID tag is courtesy of Texas Instruments.

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Figure TF13 2 Full Alternative Text

The tag is, in essence, a transponder commanded by the reader.

The functionality and associated capabilities of the RFID tag depend on two important attributes: (a) whether the tag is of the active or passive type and (b) the tag’s operating frequency. Usually the RFID tag remains dormant (asleep) until activated by an electromagnetic signal radiated by the reader’s antenna. The magnetic field of the EM signal induces a current in the coil contained in the tag’s circuit (Fig. TF13-3). For a passive tag, the induced current has to be sufficient to generate the power necessary to activate the chip as well as to transmit the response to the reader.

# Figure TF13-3 Simplified diagram for how the RFID reader communicates with the tag. At the two lower carrier frequencies commonly used for RFID communication (125 kHz and 13.56 MHz), coil inductors act as magnetic antennas. In systems designed to operate at higher frequencies (900 MHz and 2.54 GHz), dipole antennas are used instead.

Figure TF13 3 Full Alternative Text

Passive RFID systems are limited to short read ranges (between reader and tag) on the order of 30 cm to 3 m, depending on the system’s frequency band (as noted in Table TT13-1).

# Table TT13-1 Comparison of RFID frequency bands.

Table TT13 1 Full Alternative Text

The obvious advantage of active RFID systems is that they can operate over greater distances and do not require reception of a signal from the reader’s antenna to get activated. However, active tags are significantly more expensive to fabricate than their passive cousins.

# RFID Frequency Bands

Table TT13-1 provides a comparison among the four frequency bands commonly used for RFID systems. Generally speaking, the higher-frequency tags can operate over longer read ranges and can carry higher data rates, but they are more expensive to fabricate.