Passive Wireless Surface Acoustic Wave Temperature Sensing System

The SAWR sensor is a type of new sensors which is sensitive to the various physical objects and adapt to the trend of MEMS with a wide application and a good prospect for its advantages of wireless and passive. Based on the character of wireless and passive SAWR temperature sensors, this paper optimizes the set of SAWR temperature sensor and designs the reader circuits of SAWR temperature sensor system.This paper illustrates the theories of SAWR temperature sensor. According to the choice and the tangents of piezoelectric substrate materials, the TCF of SAWR temperature sensor is optimized. Therefore, this paper uses quartz as piezoelectric substrate materials to meet the need of wireless and passive SAWR temperature sensor system. The factor of SAWR temperature sensor is measured by VNA to comfirm the center frequency of SAWR temperature sensor.The recorded data of frequency-temperature during the full range is plotted to analyse the TCF of actual SAWR temperature sensor.In this paper, all parameters of helical antenna are emulated by HFSS,such as antenna diameter, screw pitch, number of turns and so on.The real helical antenna is made of copper wire. The S11 factor and SWR factor of all parts SAWR temperature sensor of are measured by VNA to analyse the performance of helical antenna.Base on the character of SAWR temperature sensor, the reader circuit of wireless and passive SAWR temperature sensor system includes transduction circuit, receiver circuit and the part of signal processing. The transduction circuit includes signal source, modulation circuit and PA circuit.The carrier signal is generated by the single PLL, which is modulated by RF switch, amplified by PA and transmitted by antenna. The receiver circuit includes pre-amplification electric circuit, mixer circuit, baseband differential amplifier circuit and ADC circuit. The echo signal of SAWR temperature sensor is amplified and smoothed by the pre-amplification electric circuit. Then, the signal is down converted by mixer circuit. ADC circuit makes analog signal into digital signal, which is amplified by baseband differential amplifier circuit. The task of the part of signal processing is to analyse the frequency spectrum by FFT after the median filter so that the temperature value is calculated according to the formula of frequency-temperature.In the last part, the paper summarizes the researches of the project, concludes the inventive points, points out the deficiencies, and expects the further work.