Surface acoustic wave (SAW) devices, used as filters or diplexers, are well established components in communications applications like in mobile handsets or television sets. The characteristics of SAW devices make them also well suitable for the application of wireless sensors or wireless identification tags. A great advantage of SAW sensors is their completely passive operation without the need for additional power supplies. SAW sensors can be designed to sense several physical or chemical quantities like temperature, pressure, stress, or gas concentration. During operation of the wireless sensor the energy delivered from an RF pulse sent by an interrogation unit is picked up by the antenna, stored in the surface acoustic wave, modified by the sensor effect and is finally transmitted back to the interrogation unit. In this paper the basic operating principles of SAW devices are reviewed and two applications, a state-of-the-art tire pressure sensor and a moisture sensor, are presented.
|