Design And Implementation Of Passive Wireless SAW Temperature Collector

It is becoming increasingly difficult to measure the parameter of temperature in the specific application occasions,such as the inner side of automotive tires,high voltage and strong electromagnetic environment in the high voltage switchgear,the place where people are not easy to get closer or enter.The traditional measurement method with thermometer far from enough to meet the current needs,so the application of passive wireless measurement methods come into being.In this environment,the temperature measurement technology of passive wireless surface acoustic wave(SAW)has been rapid development.However,the complex structure of the current SAW temperature collector and the large size of the SAW sensor pose problems for the promotion of the application.In this paper,the structure and working principle of SAW resonator are introduced while the research of generation and transmission principle of SAW are finished.The theoretical basis for the design of SAW temperature collector is provided through the study of SAW resonator which illustrated the relationship between temperature and frequency.A compact SAW temperature sensor with simple circuit which only has one frequency conversion structure is designed by the impedance matching circuit and the appropriate antenna on the basis of research on it both at home and abroad.The SAW sensor model was established and simulated after that.Then,the performance of the temperature collector was tested.The main research contents are as follows.1.the difference and characteristics of the delay SAW resonator and the resonant SAW resonator are studied.The structure and temperature frequency characteristics of the resonant SAW resonator are analyzed in detail.All of these provide a theoretical basis for subsequent design.2.The transmission circuit of generating excitation signal is studied and designed.The direct digital synthesizer(DDS)technology and the phase locked loop(PLL)are applied to generate the high precision sweep signal and the local oscillator signal,respectively.The mixer circuit is designed and the desired 433.9MHz sweep excitation signal is obtained after the mixing.The design of lumped parameter band-pass filter is studied and the band-pass filter is designed and simulated by ADS which is used to filter the difference frequency signal and the multiple harmonic signals after mixing.The two stage power amplifier circuit is designed to increase the distance of the collector.3.The receiver circuit of receiving echo signal is studied and designed.A low noise amplifier is designed to amplify the power of the received signal on the condition of low noise.Power detection technology is studied and a power detection circuit is designed in this paper.Comparing the power of echo signal,it can be seen that the frequency corresponding to the maximum power is the resonant frequency of the resonator.And according to the relationship of temperature and frequency,the ambient temperature value can be obtained.In the end,50 ohm microstrip was designed to reduce the power loss of the signal over the transmission line.4.The overall software implementation is designed.The output of sweep signal is obtained after the procedure,the accurate output of the local oscillator signal is realized after the configuration of the phase-locked loop register,and the conversion from analog power to digital signal is achieved by the ADC program.5.The PCB design is optimized based on EMC compliance technology.According to the equivalent circuit of SAW sensor,the spectral characteristics and response characteristics of SAW sensor are simulated by MATLAB which shows that the SAW sensor has good sensitivity.Finally,the collector is tested and the result shows that the accurate sweep signal with the power of 17.6 dBm could be output,the performance of receiver circuit is good and the temperature information could collected correctly.