Figure 2. Electromagnetic Coupling [2]

Introduction

RFID stands for Radio Frequency Identification. The first RFID experiments were conducted in 1948 when Harry Stockman wrote his revolutionary paper "Communication by Means of Reflected Power," but the technology has advanced considerably since then. By the 1960's, microprocessor and communications technologies had caught up enough to make RFID systems a reality. Now, RFID technology permeates our everyday lives, through things like tolltags, smartcards, and even animal identification.

So what is RFID? RFID is a system of storing and retrieving data through electromagnetic transmission to an RF compatible integrated microchip. Sounds simple, right? Let's take a closer look.

Basics of RFID Functionality
An RFID system needs two parts: a transponder and a transceiver. The transponder is a small electrical circuit (like the tag pictured on the top of this page) which can be programmed with data. There are two types of transponders: active and passive. Active transponders require an external power source, such as a small battery, to operate, while passive transponders require no power source. The transceiver is a reader with an antenna that sends a signal to transponders in order to read and/or write data to them.

Communiation between transponders and transceivers can take place in two ways, depending on how far away the transponder is away from the transciever. Transponders designed for close-range communication (usually passive devices) operate using inductive coupling produced by the transceiver's low frequency signal as a power source. The coupling produces a current in the circuit, which allows the transponder to to transmit its data (see Fig. 1).

The second type of communication takes place when the transponder (usually active) and transciever are separated by a distance greater than that of an inductive coupling signal. In this case, higher frequency electromagnetic waves are produced by the transceiver, propagate to the transponder, and the transponder sends back a signal containing its data to the transceiver (see Fig.2).

For passive devices, the signal sent from the transceiver to the transponder via inductive coupling first charges the circuit so that it can send its data back to the transceiver. Active devices that use electromagnetic signals participate in a series of signals, or handshakes. The transeiver polls for information, waits for a transponder signal to indicate that it is ready to transmit, and tells the transponder to send its message. Once the message is recieved, the transceiver sends back a confirmation message. These handshakes are used in situations where more than one transponder is available to send messages (frequency- or time-division multiple access).

Types of Transponders
RFID transponders come in all shapes and sizes and can be customized to the application for which they are designed. Some (like the one pictured beside the title of this webpage) are thin, flexible one-layer circuits that are often attached with adhesive to things like library books and retail merchandise for theft-prevention or inventory purposes. Others (like the ones pictured in Figure 3) can be as large as a hockey puck or small as a grain of rice! The versatility of RFID devices gives this technology a myriad of uses and applications.

Figure 3: RFID transponder tags [3]