Radio Frequency Identification (RFID) is an automatic identification technology emerged in the 1990s. RFID technology has a variety of bar code technology does not have the advantages of a wide range of applications, can be applied to the second generation of citizenship *, city card, financial transactions, supply chain management, electronic publication of the text fee (ETC), access control, airport baggage management, public transportation, container identification, livestock management Therefore, it becomes very important to master the technology of manufacturing RFID chips. At present, the growing application demand for RFID chips puts forward higher requirements, requiring a larger capacity, lower cost, smaller size and higher data rate. According to this situation, this paper proposes a long-range, low-power passive UHF UHF RFID transponder chip RF circuit.
RFID commonly used operating frequency including low-frequency 125kHz, 134.2kHz. high-frequency 13.56MHz, UHF 860 ~ 930MHz, microwave 2.45GHz, 5.8GHz and so on. Because the low-frequency 125kHz, 134.2kHz, high-frequency 13.56MHz system to coil as an antenna, the use of inductive bogey, its working distance is closer, generally no more than 1.2m, bandwidth is limited to a few kilohertz in Europe and other regions. However, UHF (860-93Uh1Hz) and microwave (2.45GHz, 5.8GHz) can provide longer working distance, higher data rate, and smaller antenna size, and thus become a hot research area for RFID.
The RF circuit chip proposed in this thesis is flowed by Chartered 0.35μm 2P4M CM0S process supporting Schottky diode and electrically erasable programmable read-only memory (EEPROM). The Schottky diode, with its low series resistance and forward voltage, provides high conversion efficiency when converting the received RF input signal energy into a DC power supply, thereby reducing power consumption. With an effective omnidirectional radiated power (EIRP) of 4W (36dBm) and an antenna gain of 0dB, the RF circuit chip operates at 915MHz with a read range of greater than 3m and an operating current of less than 8μA.
Passive RFID Transponder
1 RF circuit structure
Figure 1 is a system diagram of the UHF RFID transponder chip, which mainly contains RF circuits, logic control circuits and EEPROM. among them, the RF circuitry can be divided into the following main circuit modules: local oscillator and clock generation circuits, power-on reset circuits, voltage reference sources, matching networks and backscatter circuits, rectifiers, voltage regulators, and amplitude modulation (AM) demodulators, etc. There are no external components except for the antenna. Among them, there are no external components except the antenna, and the antenna part adopts a dipole structure and matches the input impedance of the rectifier through a matching network, which serves as the only energy source of the whole chip. Its equivalent model is shown in Fig. 2. The real part of the dipole antenna impedance consists of Rra and Rloss, two parts, where Rra is the radiation impedance of the dipole antenna, which is inherent to the dipole antenna and is generally 73 Ω, which characterizes the antenna’s ability to radiate electromagnetic waves externally; Rloss is the ohmic resistance brought about by the metal used in the production of the antenna, and it generally only generates heat. The imaginary part X of the antenna impedance is generally positive, this is because the antenna is generally always externally inductive, the size of this equivalent inductance generally depends on the topology of the antenna and substrate materials. The rectifier converts the coupled RF input signal power into the DC voltage required by the chip. A voltage regulator stabilizes the DC voltage at a certain level and limits the amplitude of the DC voltage to protect the chip from breakdown due to overvoltage.The AM demodulator is used to extract the corresponding data signal from the received carrier signal. A backscatter circuit is used to change the impedance of the RF circuit through variable capacitance to send transponder data to the RFID interrogator or reader. The power-on reset circuit is used to generate a reset signal for the entire chip. Unlike 13.56MHz high frequency (HF) transponders, 915MHz UHF transponders cannot derive a local clock from the frequency division of the carrier, but can only be clocked by a built-in low-power local oscillator for the digital logic circuitry. All these circuit blocks will be described in detail one by one in the following sections.
Fig. 1 System diagram of UHF RF1D transponder chip
Fig. 2 Equivalent electrical model of the transponder antenna
2 Circuit Design and Analysis
2.1 Rectifier and Voltage Regulator Circuit
In this thesis, a Dickson charge pump consisting of a Schottky diode is used as the rectifier circuit, and its circuit schematic is shown in Figure 3. This is because Schottky diodes have low series resistance and junction capacitance, which can provide high conversion efficiency in converting the received RF input signal energy into DC power supply, thus reducing power consumption. All the Schottky diodes are connected together by poly-poly capacitors, wherein the longitudinal capacitor charges and stores energy during the negative half cycle of the input voltage Vin, while the transverse capacitor charges and stores energy during the positive half cycle of Vin, thereby generating a DC high voltage which is generated as:
VDD=n-(Vp, RF-Vf, D)
Where Vp, RF is the amplitude of the input RF signal, Vf, D is the forward voltage of the Schottky diode and n is the number of stages of the charge pump employed.
Fig. 3 Rectifier circuit diagram
RFID Clothing Tag
Figure 4 shows the circuit diagram of the voltage regulator. The circuit is to stabilize the DC voltage output from the rectifier at a certain level, and provide a stable operating voltage for the entire transponder chip to ensure that the amplitude of the DC voltage will not change due to changes in the physical location of the transponder chip, to avoid possible breakdown of the chip, thus playing a role in protecting the transponder chip. The circuit uses a self-biased Cascnde structure, the reason for choosing this circuit structure is that the Cascnde structure exists in the isolation of the common gate tube, so that it has a very good power supply fluctuation suppression capability, thus improving the power supply rejection ratio (PSRR), to ensure that the two branches of the current is basically stable. The area ratio of Q1 to Q2 is 1:8. In addition, unlike general HF RFID transponders, a low-power voltage reference source with a low-voltage startup circuit is used in the design to reduce the overall power consumption of the chip.
Figure 4 Voltage regulator circuit diagram
3. Conclusion
This paper presents the design of a low-power, long-range passive UHF RFID transponder chip RF circuit based on the Chartered 0.35μm CMOS process. By utilizing Schottky diodes in the rectifier circuit, high conversion efficiency and low power consumption are achieved, enabling a read range of over 3m at 915MHz with an operating current below 8μA. The proposed RF circuit integrates key modules such as a rectifier, voltage regulator, AM demodulator, backscatter circuit, and power-on reset circuit, eliminating the need for external components except the antenna. The voltage regulator ensures stable DC output, protecting the chip from overvoltage breakdown, while the self-biased Cascode structure enhances power supply rejection. Compared to traditional HF RFID systems, this UHF design offers longer working distance, higher data rates, and a more compact structure, making it suitable for applications such as supply chain management, access control, and electronic toll collection. Future work may focus on further optimizing power efficiency and reducing chip size to meet growing demands in RFID technology.
4. Frequently Asked Questions