Abstract
With the development of industries, there is a growing need for sensitive, accurate, and inexpensive sensors of measuring relative humidity. Humidity sensors are expected for a broad spectrum of applications in meteorology, chemical industry, process control, medical instrument, agriculture, and etc. A good sensor should be characterized by high sensitivity, wide dynamic range, fast response, good reproducibility, easy interface electronics, small size, and minimum cost. It is hard for single sensing system matches all these criterions. However, surface acoustic wave ( SAW) sensing devices are promising candidates to pursue the above goals. In this thesis, the 145MHz based SAW resonator is fabricated and integrated with the amplifier to form a SAW-based oscillator. To function as a humidity sensor, hygroscopic polymer polyethylenimine is coated on the delay line of the resonator as sensing film by airbrush method. Moreover, dual delay line configuration is constructed for the common mode rejection for external influences such as temperature, drift, etc. Then, the sensing devices are exposed to repeated cycles with various relative humidity to investigate the performances including sensitivity, reproducibility, and repeatability. The results reveal good linearity between frequency shifts and various relative humidity and enough short-term repeatability.
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