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Influence of receiver noise properties on resolution of passive wireless resonant SAW sensors

V. Kalinin.

Proc. of 2005 IEEE Int. Ultrasonics Symposium, Rotterdam, Holland, 19-21 Sep. 2005, pp. 1452 - 1455.


Wireless or contactless sensors based on SAW resonators and delay lines working as passive back-scatterers have found application in the areas where temperature and mechanical strain need to be measured on rotating parts. As an example, resonant SAW sensors are used in automotive tire pressure monitoring systems (TPMS), they are being developed for torque measurements in electrical power assisted steering systems, driveline control and engine management systems.

Resolution is one of the most important characteristics of the passive wireless SAW sensor. It is mainly determined by noise in the measured value, which in its turn depends on the loaded Q factor of the SAW resonator, noise properties of the receiver and the algorithm used for spectrum estimation. Several publications investigated theoretical limit for the resolution that is set by additive noise of the receiver. It was shown that the potentially achievable resolution could be very high. For instance, the standard deviation of the measured strain can be as small as 0.02 microstrain at a distance of 1 m for the interrogation power of 10 mW at 433 MHz, the receiver bandwidth of 0.5 MHz, noise figure of 5 dB and the loaded Q = 6000 of the resonator made on ST-X cut quartz. However in reality the achievable resolution is considerably worse. This paper investigates additional limitations on the SAW sensor resolution that are imposed by the phase noise of the local oscillator of the receiver.

The problem is studied by means of stochastic simulations on the basis of information on the realistic phase noise of the receiver synthesiser used in the wireless interrogation unit. It is shown that the additive noise determines the resolution only at a distance larger than 3 m in the free space. This situation may be typical for TPMS where the received signal power can be as low as -80 dBm. The received signal in torque sensors is much stronger, typically -20...-10 dBm. In this case or in the case of shorter distances for TPMS it is mainly the phase noise that determines the resolution. For a typical value of the phase noise of -90 dBc/Hz at 10 kHz offset the predicted standard deviation of measured strain is 25 times larger than the one determined only by additive noise. This figure is much closer to experimental results.

The paper also studies variation of the dynamic range of the wireless resonant SAW sensor with interrogation distance and the number of coherently accumulated SAW responses. Practically achievable dynamic range for the torque sensor is close to 60 dB.