SAW sensors and correlative signal processing-a method providing multiple access capability

We present a method of solving the problem of access to more than one passive surface acoustic wave (SAW) sensor in the range of a single interrogation system. The sensor information of the SAW sensors are evaluated almost simultaneously by use of the correlation property of binary phase shift keying (BPSK) coded sensors. In general a SAW sensor is affected by a measurement quantity in two ways: a change of the length of the substrate and a change of the propagation velocity of the SAW. Both influences yield a time scaling of the sensor´s impulse response. If the interrogation signal is exactly the time-inverse of the sensor´s impulse response the sensor will answer with its autocorrelation function (ACF). By changing the time scaling factor s _INT of the interrogation signal one gets different correlation functions with of course different peaks. The highest peak will appear if the scaling factors of the sensor impulse response and the interrogation signal are identically. This matching scaling factor s _M provides a means of evaluating the measurement quantity. All the used sensors have the same code but different time delays. So the received signal is a succession of correlation peaks due to the different sensors. By time windowing one is able to evaluate the information of each sensor almost simultaneously. We show the principal concept, mathematical simulations, numerical estimations and finally a method to improve the system performance