Design Of Analog Front End For Passive UHF RFID Tag With Humidity Sensor

RFID (Radio Frequency Identification) has the characteristics of small volume, low power consumption, no contact information transmission and so on. And it gets more and more applications in intelligent recognition, positioning, monitoring, remote automatic testing and etc by combining the RFID in the IOT with various kinds of sensors. This paper studies the analog front-end of a kind of the UHF (ultra high frequency) RFID combined by the humidity sensor in a chip.The main work of this paper include the following aspects:1、According to the ISO18000-6 Type C protocol specification, the system structure of the passive UHF RFID humidity sensor is studied, and the electrical performance parameters of every sub module is determined.2、We design passive UHF RFID tag integrated humidity sensor analog front-end circuit on SMIC 0.18um CMOS technology. Which is consisted of a rectifier power supply, a voltage reference, a voltage regulator, a demodulator circuit, a modulator circuit, a humidity sensor, a local oscillator, a power on reset circuit. For the purpose of low power consumption, the output setting voltage to the digital circuit and analog circuit is 1.8V and 1V of charge pump of rectifier circuit. By using subthreshold MOS transistor to reduce power consumption in the reference voltage generation circuit. RFID tag output uses the backscattered working mechanism of the ASK modulated which adjusts the matching degree of the input impedance to the tag antenna,it can make the antenna back radiation of the carrier signal, then send the tag’s output to the reader, which avoid the active emission of radio-frequency carrier, besides, it is very low power consumption. The simulation results show the analog front-end system can work properly with the preceding date rate 40Kbps to 160Kbps, and the total power consumption is 7uW, so it satisfies the low power design specifications.3、We design the antenna which works on the center operating frequency of 900 MHz., and it uses the bent dipole antenna type. with the 80MHz operating bandwidth and a 50 ohms input impedance, while the VSWR is under 1.1, The antenna’s size is 130mm*30mm*1.6mm,which takes the size and the reception of the electromagnetic energy into consideration.4、We study the CMOS technology of the humidity sensor with the interdigital structure, which has the feature of good chemical stability and fast response.