Radio-frequency-identification (RFID) technology has expanded into a wide range of markets. It is particularly well suited to supply-chain management, due to the contact-less, non-line-of-sight nature of the technology. Passive RFID has been on the market at low (125 kHz) and higher (13.56 MHz) frequencies for some time, with various UHF RFID standards established prior to 2003. The Massachusetts Institute of Technology's Auto-ID Center (Cambridge, MA) recognized the problems of various proprietary RFID standards and realized that provincial protocols would impede the development and large-scale deployment of RFID technology. A single open standard was needed for an environment of interoperability and international regulatory compliance.¹ These two values formed the backbone of what they proposed as the next generation of UHF RFID—the precursor to the Gen 2 standard. With a single worldwide specification in place, UHF RFID-based systems would become faster, easier to use, and less costly; more robust; and provide a multi-supplier path going forward. The Auto-ID Center kicked off the Gen 2 effort in June 2003 at a seminal meeting in Zurich, Switzerland. They would eventually transfer the responsibility for development and commercialization of the evolving standard to EPCglobal^™ which, in December 2004, ratified the standard as "Generation-2 UHF RFID Protocol for Communications at 860 MHz -960 MHz."

Following ratification of this standard, UHF RFID integrated-circuit (IC) design activity has increased. But there are two major design constraints from an RFID IC design standpoint: power availability/bandwidth and transponder complexity.

The requirements for the UHF Industrial, Scientific, and Medical (ISM) band currently vary widely in major countries in terms of allocated spectrum, bandwidth, and radiated power, which is often stated as effective isotropic radiated power (EIRP). According to the EPCGlobal regulatory status report for UHF spectrum,² the permitting operating frequency range is the 100-MHz bandwidth from 860 to 960 MHz, with acceptable power levels to 4 W. The UHF ISM band requirements for the three different regions are as follows. For North America, the operating band is 902 to 928 MHz with 4 W maximum EIRP. For Europe, the operating band is 865 to 868 MHz with 2 W maximum-EIRP. In Japan, the operating band is 952 to 954 MHz with 4 W maximum EIRP.

Transponder complexity is another design constraint. A transponder's read range depends on the minimum turn-on power (threshold power) for the RF IC chip (tag). In a UHF RFID system, passive back-scattering is often used in the backward link, from the tag to the reader. The read range is often set by the forward link, the communication from the reader to the tag, through the radiated power available at the tag. This is because backscattered signal strength accessible at the reader RF front end is on the order of -25 to -65 dBm.

Selecting the right technology to fabricate an RFID transponder IC with the right requirements is also a challenge. To meet the low-power requirements, a Schottky point-contact diode with low turn-on voltage, low junction capacitance, and high current drivability is often desired. Since Schottky contacts are not part of a standard silicon CMOS semiconductor process, research has pursued fabricating Schottky contacts using standard (low-cost) digital bulk CMOS processes. Other efforts have involved more expensive processes, such as silicon BiCMOS, which offers additional high-speed bipolar-junction-transistor (BJT) devices, and silicon-over-insulator (SOI) technology, which offers excellent low power performance. What follows is a review of the RF circuit techniques needed to design a basic UHF RFID transponder, including critical modules such as the rectifier, modulator/ demodulator, and the digital block.

A UHF RFID transponder (Fig. 1) is comprised of four building blocks: rectifier, modulator, demodulator, and digital circuits that handle the logic-level protocol and memory function.³ In a passive RFID system, the power supply is extracted from the incoming interrogating wave. Since power is scarce, it is important that transponder power consumption be kept to a minimum.

A passive RFID transponder uses rectifier circuits to convert the coupled electromagnetic power to the DC power supply needed for the chip. Parameters that characterize rectifier circuit performance include the input impedance, Z_in or the chip's quality factor (Q), the chip operating power, P_in, and the voltage level, V_in. The rectifier circuitry must also convert incident RF energy to DC energy with the highest possible efficiency (η). A circuit designer must also consider achieving the highest possible output voltage level and input impedance in addition to maintaining high conversion efficiency.⁴ Two commonly use structures are an ordinary full-wave rectifier and a Dickson charge pump.

The full-wave rectifier is very common, using a two-diode cascaded structure. There are variants on this structure, including those based on NMOS and PMOS switches. In principal, a fullwave rectifier has good power efficiency. However, it requires that the input voltage level exceed 3 V_TH so that the chip can reach the desired output voltage. As a result, the working range of a full-wave rectifier circuit is limited for UHF RFID applications unless it is supplemented by a high-radiation-resistance antenna and very high-Q matching network to boost the incident voltage level. The antenna design is a topic for a separate article and will not be covered here. Matching networks normally have achievable Qs of order of 10 only.