This paper introduces a method of wireless read out of high Q surface acoustic wave (SAW) resonator sensors. The resonator is excited by a short RF pulse and decays after switching off the interrogating signal. In the measurement system, a gated phase locked loop (GPLL) locks to the resonance frequency of the SAW resonator within a few bursts. Then the frequency of the GPLL oscillator is synchronized to the resonance of the sensor and can be measured easily. The concept is intended to yield an alternative to interrogators with expensive signal processing. Considering the inherent limitations, the proposed system presents a low cost solution for temperature, force, torque, etc. measurements. We describe the sensors, the signals, and the implemented system. Results of temperature measurements using quartz resonators are presented, and merits and disadvantages are discussed.
DX centres, as well as other charged impurities with random location in the host crystal, give rise to fluctuations in the local potential. These fluctuations are comparable in amplitude to the level splittings of the DX centres due to different numbers of Ai neighbours in AI,Ga, _,As:Si and also to the variation in the quasi-Fermi level in experiments involving t h e kinetics of electron capture and emission processes. Thus, to describe the situtation in AI,Ga,_,As:Si in a quantitative way we develop a self-consistent model of the fluctuations. The resulting broadening of the DX level is taken into account in evaluating transport experiments. The proposed model yields the ground state energies and the barriet height for the four types of Si DX centre as functions of alloy composition and hydrostatic pressure. W e explain also t h e non-exponential behaviour of the capture kinetics as well as the mobility in t e r m s of t h e impurity level broadening and self-screening which results from a minimization of the Coulomb energy of interacting charges localized on donors.
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