Passive RFID finds unanticipated mass-market applications: part 1

Passive RFID tags have been widely adopted, and some applications show creative adaptation to unmet needs.

It’s interesting to see how advances in technology often are adapted to applications that were not highlighted in their original “use case” scenarios and how clever people adapt or leverage new technologies in unscripted ways. A good example of this phenomenon is radio frequency identification (RFID), where wireless tags, both unpowered (passive) ones and battery-powered (active) ones, have found uses that are both innovative and clever.

What is an RFID tag? It’s a small, thin, flat tag with an integral electronics assembly that contains an identity code and perhaps some additional data. It is interrogated and read by a non-contact reader at a distance of a few centimeters to several meters, depending on the tag type, design, frequency, and operating environment.

Due to their low cost, long life, and small size, passive tags are often used for a general category called “asset tracking.” Among the original applications foreseen by passive RFID advocates were the replacement of bar codes on grocery packages (didn’t happen) and in warehouses (lots of adoption there), non-contact personal ID cards for employees (also widely used), and wireless readers for credit cards and other financial cards (again, widespread adoption). In some cases, passive RFID would replace optical bar-code scanners; in others, it would replace physical contact and swipe.

Note that active tags offer a very different story and application set. For example, they are now embedded in professional soccer balls and footballs, and even the players, to precisely track their location and movement to centimeter accuracy in real-time. This requires interrogation rates and updates in the kilohertz range to provide the desired accuracy. While these uses are dramatic, they are relatively costly in both systems support and the tags, as the tag must be replaced (or the ball discarded) when the battery is depleted; wireless charging is an option but brings other technical issues).

Passive RFID basics are simple
A passive tag is an example of sporadic rather than ongoing, continuous energy harvesting, as it only extracts the energy it needs from an electromagnetic field as a source by the reader when being interrogated. The reader creates an oscillating field that is intercepted by the tag antenna, which is really more of an electromagnetic coil rather than a traditional antenna. The induced energy is rectified to charge a small capacitor which then powers the tag (Figure 1).

Figure 1. The concept of RFID energy transfer and subsequent interrogation is well-known and based on fundamental electromagnetic principles (Image: Analog Devices/Maxim Integrated).

The tag then exchanges its data with the reader using backscatter modulation.

A passive RFID tag has three primary components: the electronics of the chip itself, which has memory, power conversion, and “smarts;” the antenna; and a substrate onto which the chip and antenna are mounted (Figure 2).

Figure 2. At the core of an RFID tag is the active chip and its associated flat antenna, combined with substrate and packaging layers (Image: InfinID Technology).

The tag can be packaged in several ways: (1) Inlay, with only the RFID antenna, chip, and substrate, and there is no paper label or plastic encasement; (2) label, where a label is wrapped around the inlay (Figure 3); or (3) self-contained tag, where a protective encasement is wrapped around the inlay and transforms it into a tag which can be designed to be fairly rugged.

Figure 3. Looking at a basic tag gives little insight into the underlying electronics and packing sophistication (Image: InfinID Technology).

Some tags are designed to work on metal surfaces; in contrast, others will stop working if applied to metal surfaces. In general, the smaller the tag, the smaller the read range.

RFID is both old and new. Understanding of the mutual coupling principle is quite old, of course, and the idea of transmitting power and interrogating a passive response has been understood for decades and pursued for about 50 years. RFID systems became practical with the development of ultralow cost, tiny ICs, and their processes, which could support the multiple needed functions in a suitable super-thin package.

Once RFID moved beyond the embryonic stage, the industry established multiple standards to enhance interoperability. Among the frequencies in use are 125 kHz to 134.2 kHz, 13.56 MHz, 860 MHz to 960 MHz, and 2.45 GHz to 5.8 GHz (called microwave tags). Each band has defined operational and performance specifics, and each offers tradeoffs in range, performance, directionality, cost, and other RF attributes (Figure 4).

Figure 4. Passive RFID range is a function of operating frequency; the actual numbers vary due to electrical environment, type and density of nearby objects, and many other factors (Image: TechTarget).

The standards also define how the RFID data is formatted (the bit patterns and data fields), the range (sometimes applications want more, some want less), the read-cycle protocol, and more. Since the interrogating powers passive cards read signals, they are very power-constrained, so the protocol is critical as there is limited opportunity for multiple retries.

This article will not discuss the technical details of passive RFID; the References call out some useful sources of such information. Instead, this article will look at two interesting applications of passive RFID tags, which have reached the mass market of average consumers, and which are somewhat unexpected.