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Passive UHF RFID Transponder Design

Radio frequency identification (RFID) is a kind of automatic identification technology that emerged in the 1990s. RFID technology has many advantages that barcode technology does not have, and has a wide range of applications, which can be applied to the second generation of citizen identity*, city card, financial transactions, supply chain management, electric publication document fee (ETC ), access control, airport baggage management, public transport, container identification, livestock management, etc. Therefore, it has become very important to master the technology of manufacturing RFID chips. The growing demand for applications is now putting higher demands on RFID chips, requiring higher capacity, lower cost, smaller size and higher data rates. Based on this situation, this paper proposes a long-range, low-power passive UHF RFID transponder chip RF circuit.

RFID commonly used operating frequencies include low frequency 125kHz, 134.2kHz. high frequency 13.56MHz, UHF 860-960MHz, microwave 2.45GHz, 5.8GHz, etc. Because the low-frequency 125kHz, 134.2kHz, high-frequency 13.56MHz system to coil as an antenna, the use of inductive scourge, its working distance is closer, generally not more than 1.2m, bandwidth in Europe and other areas limited to a few kilohertz. However, UHF (860 to 960MHz) and microwave (2.45GHz, 5.8GHz) can provide longer working distances, higher data rates and smaller antenna sizes, making them a hot research area for RFID.

The RF circuit chip proposed in this thesis is fabricated using a Chartered 0.35μm 2P4M CM0S process supporting Schottky diodes and electrically erasable programmable read-only memory (EEPROM). The Schottky diode has a low series resistance and forward voltage, providing high conversion efficiency when converting the received RF input signal energy to power from a DC 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 reading distance greater than 3m and an operating current of less than 8μA.

1 RF circuit junction
The UHF RFID transponder chip consists of an RF circuit, a logic control circuit and an EEPROM, of which the RF circuit can be divided into the following main circuit blocks: local oscillator and clock generation circuit, power-on reset circuit, voltage reference source, matching network and backscatter circuit, rectifier, voltage regulator and amplitude modulation (AM) demodulator. There are no external components except the antenna, which is a dipole structure and is matched to the input impedance of the rectifier via a matching network as the sole source of energy for the entire chip. The equivalent model is shown in Figure 2. The dipole antenna impedance of the real part by Rra and Rloss, two parts, where Rra for the dipole antenna radiation impedance, is inherent to the dipole antenna, generally 73Ω, it characterizes the antenna external radiation electromagnetic wave capacity; Rloss for the production of the antenna metal brought ohmic resistance, generally only generate heat. Antenna impedance of the imaginary part of X is generally positive, this is because the antenna is generally always external inductance, the size of this equivalent inductance generally depends on the antenna topology and substrate material. The rectifier converts the coupled RF input signal power into the DC voltage required by the chip. The AM demodulator is used to extract the corresponding data signal from the received carrier signal. The backscatter circuitry is used to vary the impedance of the RF circuitry through variable capacitance to send transponder data to the RFID interrogator or reader. The power-on reset circuit is used to generate the reset signal for the entire chip. Unlike the 13.56MHz high frequency (HF) transponder, the 915MHz UHF transponder cannot be clocked locally from the carrier by dividing the frequency, but can only be clocked by a low-power local oscillator built into the digital logic circuitry section. All these circuit blocks will be described in detail in the following sections.

2 Circuit design and analysis
2.1 Rectifier and voltage regulator circuits
This thesis uses a Dickson charge pump composed of Schottky diodes as a rectifier circuit, the circuit schematic of which is shown in Figure 3. This is because Schottky diodes have low series resistance and junction capacitance and can provide high conversion efficiency when converting the received RF input signal energy into a DC power supply, thus reducing power consumption. All Schottky diodes are connected together via poly-poly capacitors, where the longitudinal capacitors charge and store energy during the negative half-cycle of the input voltage Vin, while the transverse capacitors charge and store energy during the positive half-cycle of Vin, thus generating a high DC 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 used.

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