SAW passive wireless multi sensor system

Previous work has been presented on passive wireless SAW sensors using the orthogonal frequency coding (OFC) technique. Wireless operation of such sensors has been demonstrated using a network analyzer and only one sensor at a time. A technique to allow multiple sensors to operate at the same time has been developed and demonstrated using network analyzer measurements that were added together to simulate simultaneous operation within the system. NASA is sponsoring research and development of SAW wireless, passive sensors that are environmentally robust and operate in a multisensor environment. This paper will present a first fully functioning 915 MHz interrogation system for passive wireless OFC SAW temperature sensors. The paper will present the interrogator hardware for the SAW sensors and the analytical model and how it is used to extract the temperature data from each particular sensor. Spread spectrum techniques are demonstrated that allow operation below the noise level by using processing gain techniques in a correlator synchronous receiver. The system presented has four sensors built on YZ LiNbO₃ around 915 MHz. Featured sensors consisted of wide band, eighth wavelength electrode transducer operating at the 3rd harmonic and two symmetrically placed reflector banks on either side of the transducer operating at the 2nd harmonic. A 35.192 MHz clock is used to control the digital control of the interrogator circuit and to produce the 915 MHz interrogating signal. Temperature is extracted from the sensor using an adaptive filter correlator technique. A four-sensor passive wireless system has been demonstrated. Sensors were subjected to various temperatures ranging from -40?C to 65?C. Sensors were randomly positioned within a 5cm radius of each other. The four designed sensors demonstrated excellent anti-collision properties crucial for multi-sensor operation. The system has demonstrated good code anti-collision properties. It is shown that the passive wireless SAW OFC sensor is a viable solution for environments where multiple sensors operate simultaneously.