Passive UHF RFID transponder chip RF circuit design

Radio frequency identification (radio frequency idenlificaTInn, RFID) is an 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 used in second-generation citizenship*, city card, financial transactions, supply chain management, ETC, Access Control, airports Baggage management, public transportation, container identification, livestock management, etc. Therefore, it becomes very important to master the technology of manufacturing RFID chips. At present, the increasing application demands have put forward higher requirements for RFID chips, requiring larger capacity, lower cost, smaller size and higher data rate. According to this situation, this paper proposes a long-distance, low-power passive UHF RFID transponder chip RF circuit.

Common operating frequencies of RFID include low frequency 125kHz, 134.2kHz, high frequency 13.56MHz, ultra-high frequency 860-930MHz, microwave 2.45GHz, 5.8GHz, etc. Because the low frequency 125kHz, 134.2kHz, the high frequency 13.56MHz system uses the coil as the antenna, and adopts the method of inductive coupling, the working distance is relatively short, generally not more than 1.2m, and the bandwidth is limited to several kilohertz in Europe and other regions. But UHF (860～93Uh1Hz) and microwave (2.45GHz, 5.8GHz) can provide longer working distance, higher data rate and smaller antenna size, so it has become a hot research field of RFID.

The RF circuit chip proposed in this paper is tape-out using Chartered 0.35μm 2P4M CMOS process supporting Schottky diodes and Electrically Erasable Programmable Read-Only Memory (EEPROM). Schottky diodes have low series resistance and forward voltage, and can provide high conversion efficiency when converting received RF input signal energy into DC power supply, thereby reducing power consumption. When the effective isotropic radiated power (EIRP) is 4W (36dBm) and the antenna gain is 0dB, the RF circuit chip operates at 915MHz, the reading distance is greater than 3m, and the operating current is less than 8μA.

1 RF circuit structure

The UHF RF1D transponder chip, which mainly includes a radio frequency circuit, a logic control circuit and an EEPROM. Among them, the radio frequency circuit part can be divided into the following main circuit modules: 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, etc. There are no external components except the antenna. The antenna part adopts a dipole structure and is matched with the input impedance of the rectifier through a matching network as the only energy source for the entire chip. Its equivalent model is shown in Figure 2. The real part of the impedance of the dipole antenna consists of Rra and Rloss, where Rra is the radiation impedance of the dipole antenna, which is inherent to the dipole antenna, generally 73Ω, which represents the ability of the antenna to radiate electromagnetic waves; Rloss The ohmic resistance brought about by the metal used to make the antenna generally only generates heat. The imaginary part X of the antenna impedance is generally positive, because the antenna is generally inductive to the outside, and the size of this equivalent inductance generally depends on the topology of the antenna and the material of the substrate. The rectifier converts the power of the coupled RF input signal into the DC voltage required by the chip. The voltage regulator stabilizes the DC voltage at a certain level and limits the magnitude of the DC voltage to protect the chip from breakdown due to excessive voltage. The AM demodulator is used to extract the corresponding data signal from the received carrier signal. The backscatter circuit transmits the transponder data to the RFID interrogator or card reader by changing the impedance of the RF circuit through variable capacitance. The power-on reset circuit is used to generate the reset signal of the whole chip. Unlike the 13.56MHz high frequency (HF) transponder, the 915MHz UHF transponder cannot obtain a local clock by dividing the frequency from the carrier, but can only provide a clock for the digital logic circuit part through a built-in low-power local oscillator. All these circuit blocks will be explained in detail one by one below.

2 Circuit Design and Analysis

2.1 Rectifier and Voltage Regulator Circuits

In this paper, the Dickson charge pump composed of Schottky diodes is used as the rectifier circuit. The schematic diagram of the circuit is shown in Figure 3. This is because Schottky diodes have low series resistance and junction capacitance, which can provide high conversion efficiency when converting received RF input signal energy into DC power supply, thereby reducing power consumption. All Schottky diodes are connected together by poly-poly capacitors. The vertical capacitors charge and Store energy during the negative half cycle of the input voltage Vin, while the lateral capacitors charge and store energy during the positive half cycle of Vin to generate DC. High voltage, the resulting voltage is:

VDD=n·(Vp, RF-Vf, D)

Where Vp, RF is the amplitude of the input radio frequency signal, Vf, D is the forward voltage of the Schottky diode, n is the number of stages of the charge pump used.

Stabilize the DC voltage output by the rectifier at a certain level, and provide a stable working voltage for the entire transponder chip to ensure that the DC voltage amplitude will not change due to the physical position of the transponder chip, and avoid possible chip shocks. wear, so as to protect the transponder chip. The circuit adopts a self-biased Cascnde structure. The reason for choosing this circuit structure is that the Cascnde structure has the isolation effect of the common gate tube, which makes it have a good ability to suppress power fluctuations, thereby improving the power supply rejection ratio (PSRR). To ensure the basic stability of the two branch currents. The area ratio of Q1 and Q2 is 1:8. In addition, unlike general HF RFID transponders, we have adopted a low-power voltage reference source with a low-voltage start-up circuit in the design to reduce the overall power consumption of the chip.