Modelling of Wireless Temperature Sensors Based on Coded SAW Delay Lines for RF Identification

Wireless identified Surface acoustic waves (SAWs) sensors operating in the real-time offer a wide range of applications. The novelty of this paper is the use of two-dimensional (2D) COMSOL FEM modelling of two types of one-port reflective SAW delay lines (DL): a simple reflective one designed with conventional Bragg reflectors and a coded reflective one that employs a specific spatial distribution of Bragg mirrors to encode the SAW electrical response for radio frequency identification (RFID). The SAW structures are made of Al/AlN/Si stratified layers, forming respectively the IDT electrodes, the piezoelectric layer, and the substrate. The geometry and materials are selected to achieve a SAW device operating frequency around 441.2 MHz. The reflection scattering parameters of all the studied structures are calculated in the frequency domain S₁₁(f) and transformed to the time domain S₁₁(t) by performing an inverse Fourier transform (IFFT). In the coded reflective delay line structure, eight Bragg systems containing eight reflectors each are employed to generate the 10010110 code. This structure is tested as a temperature sensor; its normalized sensitivity in the temperature range of −25°C to 200°C is evaluated at −25.39 ppm/°C. The tag sensor's identification procedure is performed by using the cross-correlation technique on two RFID temperature sensors interrogated simultaneously: namely, one coded 10010110 and another one coded 11010111.