Effect of frequency tolerance and temperature response on the performance of surface acoustic wave sensor

The main requirements of a frequency output surface acoustic wave (SAW) sensor are: high measurement resolution and compatibility with digital processors. This paper presents the technical analysis of a sensitive SAW sensor for the measurement of pressure and temperature. The sensor is created by a long SAW delay path on the surface of a thin diaphragm which contacts the media to be measured. The delay path and associated electroacoustic transducers are utilized as the feedback circuit in a miniature electronic oscillator having a frequency of 515 MHz. Since the time delay varies almost linearly with diaphragm temperature and pressure induced strain, the oscillator frequency is also a predominantly linear function of pressure and temperature. The effect of the frequency tolerance and the temperature response, is the mounting stress arising from baseplate bending during the welding process. This effect can severely distort the parabolic shape of the temperature curve which may be accomplished by a much larger frequency shift. Therefore the application of a mounting technique that avoids the creation of surface stresses in the substrate during welding is essential.