Radio Frequency Identification: Evolution of Transponder Circuit Design

Radio frequency identification (RFID) is an area of automatic identification that has quietly been gaining momentum in recent years and is now being seen as a radical means of enhancing data handling processes, complementary in many ways to other data capture technologies such as bar coding. RFID systems involve the use of robust tags that permits the operator to collect data and manage it in a portable, changeable database. A basic RFID system consists of three components: an antenna, a reader and a transponder, widely known as a tag. The antennas, attached to the reader and the tags, are the conduits between the two, which control the system’s data acquisition and communication. The tag contains an electronic microchip, as shown in Figure 1, which is fabricated as a low power integrated circuit (IC). Depending on the device application, the tag memory may comprise ROM, RAM, non-volatile programmable memory (EEPROM, Flash) and data buffers.

Transponders are being developed and employed at different frequencies, ranging from the low frequency region of 125 kHz to the microwave region of 5.8 GHz to satisfy a wide range of applications. The applications include access control, automatic data collection, livestock tracking, inventory management and factory automation.

In the retail and pharmaceutical industries, losses arising from drug counterfeiting, overstocking or outdated products make RFID a sought after automatic identification system. Each year, retail and pharmaceutical markets are burdened by more than $2 B in product returns. In 2001 alone, the industry faced some 1300 recalls. To add more damage to the market, up to seven percent of all drugs in the international supply chain may be counterfeit.1 Therefore, RFID technology’s ability to track, trace, authenticate and uniquely identify each pharmaceutical product helps to improve inventory management through the reduction of out-of-stock items and increase the safety of the stock, more efficient product recalls, improvements in product quality and drug safety, and anti-counterfeiting. As the pharmaceutical industry relies upon the integrity of many forms of data throughout the process of drug trials, manufacturing, distribution and retail sale, RFID will prove to benefit the pharmaceutical industry beyond the fight against counterfeiting of drugs. Some studies estimate that RFID-based solutions could save the industry more than $8 B by the end of 2006. At the item level, RFID can provide 100 percent visibility of inventory no matter where it is in the supply chain, making it easier and quicker to move goods to the right place within the chain. The technology can improve productivity in shipping and receiving, reduce touch labor, increase the assurance of shipping and dispensing accuracy, and expand product availability at the retail store, thus reducing customer complaints and back charges. Therefore, the US Food and Drug Administration (FDA) recommends the widespread use of RFID in the pharmaceutical supply chain at the item level by 20072 to monitor the international drug supply from “manufacture to medicine cabinet,” thus keeping the drug supply safe and secure. Pharmaceutical companies like Purdue Pharma LP, maker of the painkiller OxyContin, have begun using RFID technology. The OxyContin bottles with the tags can help authorities and the company to battle against theft as the stolen bottles can be traced back to the particular pharmacy, once the thieves are caught. The maker of one of the most counterfeited pharmaceutical products, Pfizer, had planned to add passive RFID chips to bottles of the impotency drug Viagra by the end of the year 2005. A third US pharmaceutical company, GlaxoSmithKline, said it will also begin using RFID on one of its products in the next 12 to 18 months. In the United Kingdom, six manufacturers are working together on a pharmaceutical RFID and bar-code pilot to see if the technologies can be used to detect dispensing errors and counterfeit drugs before they reach the patients.3,4

RFID can also be potentially used in library systems. The Rose Creek branch of the Sequoyah Regional Library System has become the first public library in Georgia to adopt this advanced technology for materials management, employing RFID transponders to track books. With this system, patrons are enjoying self-checkout, shorter checkout lines and improved customer service, while the staff is spending more constructive time assisting patrons and less time with repetitive, clerical tasks. The system has a built-in security component that alerts the staff when items are removed from the library without authorization. Since RFID transponders need not be seen in order to be read, the library will be able to track its materials regularly by waving a portable RFID inventory wand-reader along the base of shelves. Besides, materials can be stacked up on the counter and can be identified simultaneously by the built-in RFID reader without having to scan it individually. In the same step, the security mechanism inside each tag is deactivated. It is anticipated that the system’s speed and user-friendly features will mean that the vast majority of the branch’s circulation will soon be handled without staff intervention. The Sequoyah Library System’s state-of-the-art RFID was developed by Vernon Library Supplies Inc., a Norcross-based company, in partnership with Tagsys, a world leader in the manufacture of RFID tags and hardware for libraries.5

Other developing countries like Malaysia are also adopting this advanced technology in their automatic identification system. The Malaysian government bought the RFID 0.25 mm2 chip, known as the MM chip,6 designed by the Japanese R&D company FEC Inc. While the usage of the chip is not determined, it would most probably be used in national passports and currency notes, for example, to eliminate counterfeiting activities and to greatly prevent the possibility of terrorist acts. Further research and modifications can make the chip useful for a broad range of applications, from fighting forgery to killing cancer cells. The MM chip’s potential has also attracted the attention of many countries that are looking forward to utilize the RFID technology in national applications. Government officials in China, Canada, Mexico and Australia were keen to explore the chip’s usage in a variety of projects. According to the CEO of FEC Inc., Mexico had conveyed interest in using the chip as IDs for voters in elections, while Canada and Australia wanted to use it for national identity cards.7,8

The main purpose of this article is to review and discuss the various circuit designs of RFID transponders performed by various individuals and organizations, mainly in the last three years. The advances, approaches and improvements in the designs will be examined and, if possible, compared with one another.

Architectures of RFID Transponders

Tags are either passive or active. Active tags are powered by an internal battery while passive tags derive the power supply from the field generated by the reader through the antenna. Active tags allow greater communication ranges, better noise immunity, higher data transmission rates and do not require a higher powered reader than passive devices. The trade-offs of active tags are greater size, greater cost and a finite lifetime, compared to the virtually unlimited operational lifetime of the passive tags.1

A tag is a data-carrying component to be attached on an item and needs to be robust so that the reader can easily track it. Tag designs are the most commonly explored area of RFID systems. Throughout the years, various RF researchers came up with different designs to suit different applications. Beginning from the year 1993, different ways to produce low cost and low power tags can be seen and, along the way, additional features are being added to the designs to make them more valuable.

Raymond Page, a design engineer for Wenzel Associates, emerged with the grand prize in the 1993 RF Design Awards Contest, with his winning design of a low cost and low power tag, applied to rail car tracking.11 He uses an unusually simple method of converting the interrogating RF field into a data-modulated signal, which can be transmitted back to the reader. This contributes to the low manufacturing cost of the tag design. To achieve the lower power requirement from the reader, the tag design consists of only one inexpensive microwave semiconductor, a diode, and allows all parts to be mounted on an FR-4 printed circuit board with the patch antennas. In contrast, other design approaches using expensive microwave parts, including SAW devices, oscillators, mixers, filters and amplifiers, involve more RF circuitry and tend to be power hungry, which require higher RF interrogation fields. Besides that, the tag’s efficiency in rectification, frequency doubling and modulation eliminates the need for higher power. One watt of power, transmitted with an antenna gain of 31.6 (16 dB) and received with an antenna gain of 2 (3 dB), allows the tag to function from as far as 20 feet away. This means that just over 1 mW is adequate to energize the tag.

In 1995, Pobanz and Itoh12 designed a novel microwave RFID tag in the form of an ID card that has been developed for remote identification of personnel and articles. The tag is compatible with interrogation frequencies in the range of 4 to 7 GHz, which includes the 5.8 GHz ISM band. Based on a sub-harmonically pumped quasi-optical mixer, the tag is activated by a C-band interrogation beam to up-convert and radiate a digitally modulated identification tone at two X-band frequencies. The advantages of this scheme is that the new microwave frequencies of the response signal are created non-harmonically, an octave apart from the interrogation signal without a microwave oscillator, compared to the frequency doubling in the response signal by Page.11 This scheme further avoids the interference of signals from the interrogation band and the problem of false detection resulting from an interrogator receiving reflections of its own transmitted harmonics is avoided. The quasi-optical structure consists of an anti-parallel Schottky diode pair mounted at the terminals of a planar bowtie (triangular dipole) antenna, allowing it to receive the interrogation signal and transmit the response an octave apart.

The same year, Kaiser and Steinhagen13 designed a low power passive read/write tag, following the TIRIS specification, working at a carrier frequency of 134.2 kHz. Kaiser and Steinhagen employed an LC tank, as shown in Figure 2, for wireless communication purposes and to retrieve the power to supply the microchip. The LC tank consists of the antenna LR and capacitor CR to retrieve the energy from the reader unit during the charge-up phase and to send back data during the transmission phase. The supply capacitor CL stores the energy during the charge-up phase and delivers the current to activate all circuits in the microchip when the data transmission phase begins. This technique of retrieving energy from the reader can be seen in many designs thereafter. The system works in half-duplex mode. Thus, the powering and data transmission phases are separated and this gives a superior performance at reading distances of up to two meters, depending on the size of the antenna and the allowable field strength.

Several building blocks included in the design are the ‘end of burst’ detector, RF limiter, a trimming block to allow optimal tuning of the LC tank with on chip capacitors so that the maximum possible transmission distance can be achieved, EEPROM with a shift register, a discharge circuit, a clock regenerator, a clock divider and a modulator. However, if more than one tag enters into the electromagnetic field at the same time, the one that is read is the one closest to the reader’s antenna. This is not suitable for applications whereby many tags need to be read at the same time.

Using this design, Wu, Yang and Liu14 presented a Texas Instruments compatible passive tag, operating at a frequency of 134.2 kHz. This design also has an LC tank. The differences in the design lie in the use of a digital CMOS circuit as the major part of the design and the output frequencies are implicitly determined, independent of the load of the antenna.

To overcome the problems caused by many tags entering the electromagnetic field at the same time, and to be able to read all of them at a time, an anti-collision scheme is employed into the tag design. This feature is described by Masui, Ishii, Iwawaki, Sugawara and Sawada.15 The design is a read/write 13.56 MHz tag, which includes complex functions such as anti-collision and authentication, controlled by a dedicated central processing unit (CPU). As the addition of the complex functions increases power consumption, the CPU is there to control the power consumption, therefore retaining the tag chip’s low power consumption and achieving a higher data rate, exceeding 100 kbps, than the previous work by Kaiser and Steinhagen.13

A multifunction tag, shown in Figure 3 and operating at 125 kHz, was unveiled by the Fraunhofer Institute of Microelectronics Circuits and System (IMS) in November 2000.16 This tag can be used either in the passive or active mode. In the passive mode, the LC tank formed by the inductor and capacitor serves as the RF interface to receive energy as described before. In the active mode, the tag is supplied with energy from a rechargeable battery or primary cell, which is built into the tag. The tag can also be added with an anti-collision feature and is capable to be read up to about 50 tags at a time.

In 2001, Panitantum, Yordthein, Noothong, Worapisher and Thamsirianunt17 presented a passive read-only RFID tag, targeting applications such as animal and asset tracking. It operates at 13.56 MHz and employs a Zener-zap one-time programmable (OTP) ROM to store ID. The Zener-zap technology provides an opportunity for programming an OTP at the wafer probe or field programming after chip packaging. This tag also has an anti-collision feature but instead of adding a CPU to minimize power consumption, the anti-collision feature is separated from the main power circuitry and its power utilization is minimized by a clock extractor that is subsequently divided by a factor of 32. This design also employs an RF limiter, as seen in the design by Kaiser and Steinhagen,13 to protect damages caused by CMOS gate-oxide breakdown due to excessive coupling voltage from the reader for a short distance operation. Besides that, an LC tank is used for powering and data communication purposes.

In mid-2001, Hitachi released one of the world’s smallest IC chips, known as the ?-Chip,18 which is only 0.4 × 0.4 mm in size. It is thin enough to be embedded in paper. The ?-Chip is integrated with a 2.45 GHz high frequency analog circuit and a 128-bit ROM. The chip data is recorded in its memory during the semiconductor production process, and therefore cannot be rewritten, thus providing high resistance to tampering and guaranteeing its authenticity.

In 2003, Baude, Ender, Kelley, Hasse, Muyres and Theiss of the 3M Co. in the US presented an organic semiconductor RFID tag.19 An organic semiconductor offers a lower manufacturing cost compared to its silicon counterpart, which is in favor to the current need for lower cost to encourage RFID applications in the retail market. Another advantage of an organic semiconductor is its compatibility with flexible substrates. In this design, the chosen organic semiconductor is pentacene because of its relatively high mobility, of the order of 1 cm2/V-sec. The tag is a 1-bit pentacene-based RFID tag. The tag circuitry is patterned entirely using flexible polymeric shadow masks with 20 ?m gate lengths and 30 ?m design rules. The tag operates at RF frequencies of 125 kHz and as high as 8.8 MHz. The circuits, fabricated on glass substrates, were optimized for AC powering. Unlike the other tag designs, this tag circuitry doesn’t need a separate rectification stage as it is powered directly from the resonant LC tank. Figure 4 shows the circuit schematic of the 1-bit pentacene-based tag. The circuit consists of a seven-stage ring oscillator, a NOR gate and two output buffers. The power is coupled into the circuit using inductive coupling from the reader loop to the tag loop. The ring oscillator and NOR gate were used to create a pulse signal that was in turn buffered and sent to a large inverter. This inverter was used to modulate the absorbed RF energy and the modulation was detected internally using simple peak-detection demodulation circuitry. In this design, the tag exhibited sufficient amplitude modulation of the absorbed RF to be detected externally with peak-detection demodulation.

In the same year, Applied Digital Solutions demonstrated its sub-dermal VeriChip personal verification microchip and its new, implantable, temperature-sensing microchip based on the former.20 Each VeriChip product is about the size of a grain of rice and contains a unique verification number, which is captured and detected by a proprietary handheld scanner. The VeriChips are available in several forms, some of which will be able to be inserted under the skin.21 The first sub-dermal VeriChip chipping procedure was performed on September 24 of the same year, in New York City, at the Applied Digital Solutions lower Manhattan Authorized VeriChip Center.22 The procedure to implant the tag under the skin is quick, simple and painless. The brief outpatient chipping procedure lasts just a few minutes and involves only local anesthetic. The standard location of the microchip is usually in the triceps area between the elbow and the shoulder of the right arm. Once inserted, the VeriChip is inconspicuous to the naked eye. Therefore, unlike traditional forms of identification, VeriChip cannot be lost, stolen, misplaced or counterfeited. VeriChips can be used in a variety of applications such as security, financial and emergency identification.

To achieve higher data rates, longer reading distances and smaller antenna size, Karthaus and Fischer designed a passive tag IC in the UHF band region (868/915 MHz and 2.45 GHz).23 In order to increase the reading distance, they used a different voltage generator topology and employed a very well suited technology, which is the two metal, two-poly, 0.5 ?m digital CMOS process, to support the EEPROM and Schottky diodes. The specially designed Schottky diodes, with low series resistance, allow a high efficiency conversion of the received RF input signal energy to DC supply voltage. Therefore, a low power of 16.7 ?W is sufficient to power the tag. The IC’s power supply is taken from the energy of the received RF electromagnetic field with the help of a Schottky diode voltage multiplier. The IC includes the power supply generation, phase shift keying backscatter modulator, pulse width modulation demodulator, EEPROM and logic circuitry including some finite state machines handling protocol, used for wireless write and read access to the IC’s EEPROM and for the anti-collision procedure.

By the end of 2003, Trolley Scan had developed its new energy-efficient UHF RFID passive EcoTag that uses only 250 ?W of energy to operate,24 which is about one quarter of the energy needed by the previous version of the EcoTag. Although the power consumption is not as low as seen in the work done by Karthaus and Fischer, it is considered energy efficient compared to some other UHF tags in the market. The EcoTag can operate as far as nine meters, which is a significant distance. Trolley Scan achieved such low power consumption by working on improving the common problem of transferring the energy efficiently from the air to the electronic circuit due to incompatibilities between the microchip and the antenna in the UHF band. The improved performance comes from calculating and implementing the correct transfer network to match the antenna impedance to the equivalent load of the tag circuit as seen via the power rectification circuits of the tag. Trolley Scan has since been awarded a US patent for its work.

RFID adopters and those who are skeptical in using the RFID technology often raised the privacy issue concerning the used of the technology. This called for a blocking technique that has been in its research stages at RSA Security since early 2004. Without any disruption to normal RFID operation, the RFID blocker tag25 can prevent readers from performing unwanted scanning and tracking of people or goods. The blocker tag, similar in size and cost to a normal RFID tag, works by disrupting the transmission of information to any unauthorized RFID readers. When the blocker tag is removed from a product, the RFID tags would work normally, but when RFID tags are in the coverage area of a blocker tag, the RFID tag and its contents will be shielded from the reader.

In March 2004, Toppan Forms, a technology subsidiary of Japan’s Toppan Printing, teamed with Kanazawa, a Japan-based integrated circuit manufacturer FEC Inc., to create the first multi-frequency RFID microchip, called the MM chip,26 that can operate at all frequencies from 13.56 MHz to 2.45 GHz. The aim for this chip is to make it possible for companies to use tags that can be read anywhere, regardless of local regulations. The write-once passive chip is rather small, measuring 0.5 × 0.5 mm in size, and has 32 bytes of memory storage. The chip uses a proprietary method of communication between the tag and reader, so standard readers cannot read it. Unlike most RFID chips, which are tuned to one frequency for reading and writing, a special reader developed by Toppan, using any frequency, will be able to read the MM chip. The data is written on the chip, using the infrared portion of the electromagnetic spectrum. The MM chip can store a 64-bit or 86-bit EPC.

In December 2004, a startup company named Sandtracker came up with a new RFID technology that replaces silicon chips in the tags.27 The proposed RFID tag uses a quartz crystal diode, instead of a silicon chip, for transmitting the signal from the tag to the reader. As a result, the signal is reportedly stronger and the tag offers a longer reading range and costs less to manufacture than tags using conventional RFID technology. About 4000 Sandtracker tags can be read simultaneously in any spatial orientation with read ranges of up to 18 meters. The tags can be passive or active. They contain prewritten RFID codes and can have numbers written to them at the point of use or can be written to more than once, depending upon the application and the type of Sandtracker tag that is used. The tags can operate at the recommended frequencies ranging from 100 MHz to 1 GHz or can be configured to operate at multi-frequencies. Their design hold the same data constructs, use the same frequencies and show the same signal characteristics as UHF tags based on existing EPCglobal standards. The tags also hold an EPC-compliant RFID number in the standard format.

Tag designs continue to evolve as the world is turning to new applications, performance improvement, cost reduction, low manufacturing cost and size reduction. The tag’s key design goal is to include all the required components on a single IC and use inexpensive components for cost reduction purposes. The second design goal is to minimize the power consumption to enable operation at maximum range. This requires special low voltage and low current design techniques and a suitable low power process. The third design goal is to minimize the chip area, thus requiring a small geometry process for minimal cost. Other goals include improving the performance of the tag in terms of reducing errors in detection and adding anti-collision feature. Privacy and security concerns further encourage researchers to add more security features in the tags. The designs of tags vary based on their application, operating frequency, technology used and simplicity of the designs.

Conclusions and Recommendations

Improvements in RFID are constantly taking place, and are clearly driven by the advances in RF technologies. These include the design and technology, manufacturing methods and the frequency spectrum allocation. Smaller chip surfaces will bring new assembly technologies, such as flip-chip technology.28 A use-once-and-throw-away, low cost, read-only RFID tag will eventually be possible in the near future. Any environmentally friendly material is possible to receive today’s RFID electronics. When recycled, the remains would consist of paper fibers, silicon (sand), copper and minute traces of aluminum. However, some fundamental changes in the existing RFID technology are required. These include the way the tags are manufactured and the process used.

As the demand for wireless communication and high speed mixed-signal systems continues to increase, providing sufficient electrostatic discharge (ESD) protection for the tags could be researched and implemented in the chip design. Currently, most of the existing tag chips do not contain an ESD protection circuit that is tailored towards mixed-signal circuits.

References

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2. “A Basic Introduction to RFID and Its Use in the Supply Chain,” White Paper, LARAN RFID 2004, April 2005, http://www.laranrfid.com/modules/movie/scenes/home/index.php?fuseAction=pages&rubric=pages&article=downloads.

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15. S. Masui, E. Ishii, T. Iwawaki, Y. Sugawara and K. Sawada, “A 13.56 MHz CMOS RF Identification Transponder Integrated Circuit with a Dedicated CPU,” IEEE International Solid State Circuits Conference Proceedings, February 15–17, 1999, pp. 162–163.

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17. Panitantum, A. Yordthein, W. Noothong, A. Worapisher and M. Thamsirianunt, “A CMOS RFID Transponder,” International Symposium on Communications and Information Technology (ISCIT), Pattaya, Chonburi, Thailand, 23-25, October 2002.

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Faisal Mohd-Yasin received his BSc and MSc degrees in electrical engineering from George Washington University, Washington, DC, in 1999 and 2001, respectively. He is currently a lecturer on the faculty of engineering, Multimedia University, Malaysia. His current research interests include mixed-signals and RFIC design, and MEMS.

Mei Khaw Kum received her BSc degree in computational physics and electronics from the University of Malaya, Kuala Lumpur, in 2003. She is currently pursuing her M.EngSc degree at Multimedia University, Malaysia. Her research interests include analog/digital VLSI design, RFID systems and ESD protection design.

Mamun Bin Ibne Reaz received his BSc and MSc degrees in applied physics and electronics from the University of Rajhashi, Bangladesh, in 1985 and 1986, respectively. Since 2001, he has been a lecturer on the faculty of engineering, Multimedia University, Malaysia, involved in teaching, research and industrial consultation. He is the author or co-author of more than 40 papers on design automation and IC design for bio-medical applications.