Modelling And Study Of Passive Wireless Impedance-loaded Sensors Based On SAWRs

Passive Wireless SAW Sensors(PWSS)are considered the most promisingcandidates in place of active wireless sensors so far.However,changing the layout plan,orusing different SAWs forms seems not to make any further progress for the expansion ofthe application scope.The SAW's physical properties determine that it is the best methodto apply the physical to the transmission path of the SAW.But some are not suitable fordirect measurement,which restrict the application area of PWSSs.Taking conventionalimpedance sensors as the loads of SAW devices,the new sensors which are named thePassive Wireless SAW Impedance-loaded Sensors(PWSIS)can be constructed.It is a newway to expand the sensing scope of PWSSs.Therefore,the PWSISs based SAW resonator(SAWR)needs to be extensively investigated and thereof the theoretical and applicationvalues are significant.In this paper,the current status of PWSSs and research progresses are reviewed in thepreface.Then,combining the from-inside-out idea and the spline interpolation method,theparameters of lumped-element equivalent circuit are extracted.The obtained results are asfollowing:the relative errors of series and parallel resonant frequencies of the mainresonance peaks are zero,and the relative error of series resonant conductance is 0.135%;the maximum relative error of the second-order resonant frequency is 2.88ppm,and that ofthe resonant conductance is 3.67%.In the 2 MHz frequency range around resonant peaks,the results of measurement and calculation have well consistency.On the base of extraction parameters,the best-matched matching network is taken asthe study objective.Focus on the overall admittance characteristics,the steps to obtainstable low-mismatched matching network include matching the SAWR to equivalentseries RLC branch with two reactive components and connecting the impedance sensor.The study shows that it is necessary for the impedance sensor to take the location of X₃ as it's.For a capacitance range from 0.6 to 7.5 pF,the matching network can change thecenter frequency about 270 KHz,and the SWRs are all below 1.015.The characteristicsare maintained in the structure of dual-SAWR.The average value of measured SWR isbelow 1.2.The T-CLC low-mismatched matching network requires that the initial capacitance ofthe impedance sensor is small and the relative change is large.In this paper,on the base ofthe clamped-double-membrane model,using the center-island electrode structure,andcontrolling the electrode / diaphragm radius ratio m can meet previous requirements.Study shows that the capacitance ratio will decline with the increasing of m when the ratioof displacements of the two vibrating membrane and the initial distance is fixed.Whenm=0.37,the capacitance ratio is 10.Taking the obtained data of capacitance-pressure intothe previous low-mismatched matching network,the relation curve of center frequency-pressure has good linearity.Finally,the two-port network model of transmitting and receiving antennas isintroduced into the PWSIS unit,and the complete model which can examine the effects ofantenna distance on the center frequency is constructed.Based on the model,the influencemechanism is discussed,and a measure to weaken the impact is put forward.It ismatching the self-impedance of antenna to the characteristic impedance of transmissionline.The experimental results show that the sensitivity of center frequency-antennadistance is-1.75 ppm/cm.The results are obtained on the single-SAWR construct and adistance range about 20 cm.