Radio Frequency Identification (RFID) technology is widely used across various industries, particularly in supply chain management. Its non-contact and non-line-of-sight characteristics make it ideal for tracking and managing goods efficiently. Passive RFID has been available for low frequency (125 kHz) and high frequency (13.56 MHz) applications for many years. Before 2003, multiple UHF RFID standards were developed, but the lack of a unified protocol created challenges for widespread adoption. The MIT Auto-ID Center recognized these issues and emphasized the need for an open, international standard to ensure interoperability and global compatibility.
To address this, they proposed the development of a next-generation UHF RFID standard, which eventually became known as Gen 2. This standard aimed to enhance system performance by making UHF RFID faster, more user-friendly, cost-effective, and reliable. It also paved the way for multi-vendor solutions. In June 2003, the Auto-ID Center introduced the concept of Gen 2 at a seminar in Zurich, Switzerland. Later, the standard was transferred to EPCglobal, which officially approved it in December 2004 as the "860 MHz to 960 MHz second generation UHF RFID communication protocol."
Spectrum allocation, bandwidth, and radiated power requirements vary significantly across different regions. For example, in North America, the UHF ISM band operates between 902 MHz and 928 MHz with a maximum EIRP of 4W. In Europe, the band is narrower, ranging from 865 MHz to 868 MHz, with a lower maximum EIRP of 2W. Japan’s UHF ISM band is even more restricted, operating between 952 MHz and 954 MHz with a 4W EIRP limit. These variations pose challenges for global RFID deployment.
Another key constraint in RFID design is transponder complexity. The read range of a passive tag depends on the minimum conduction power required by the RF IC chip. In UHF RFID systems, backscattering is commonly used for communication from the tag to the reader. However, the readable range is often limited by the power available in the forward link from the reader to the tag, as the backscattered signal strength is typically between -25 dBm and -65 dBm.
A basic UHF RFID transponder consists of four main components: a rectifier, a modulator, a demodulator, and digital circuitry that manages protocol and memory functions. The rectifier plays a critical role in converting the received RF energy into DC power for the chip. Key performance parameters include input impedance, operating power, and input voltage. The goal is to achieve high conversion efficiency while maintaining a stable output voltage.
Common rectification structures include full-wave rectifiers and Dickson charge pumps. Full-wave rectifiers are efficient but require a higher input voltage, typically above 3Vth, which limits their effectiveness in low-power applications. To overcome this, designers often use high-Q matching networks or specialized antennas to boost the input voltage. On the other hand, Dickson charge pumps offer a flexible alternative, especially when working with low threshold voltage MOSFETs and Schottky diodes. They allow for adjustable output voltages and can be optimized for specific applications.
The Dickson charge pump structure is particularly useful in RFID chips that include non-volatile memory, as it can generate the high voltages needed for programming EEPROM cells. While it offers advantages in flexibility, it may have lower efficiency due to leakage currents and parasitic effects. Engineers must carefully balance performance, power consumption, and complexity when designing these circuits.
In addition to the rectifier, the modulator and demodulator play essential roles in signal processing. The modulator encodes data onto the RF carrier wave, while the demodulator extracts the information from the received signal. These components must work together seamlessly to ensure accurate and reliable communication between the tag and the reader.
Finally, the digital block handles protocol execution and memory operations. It ensures that the tag follows the correct communication sequence and stores necessary data securely. As RFID technology continues to evolve, the integration of advanced digital features will further enhance its capabilities in real-world applications.
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