High-frequency stability oscillator for surface acoustic wave gas sensor(<Special issue>Commemoration of the Japan-China Joint Conference on Acoustics 2007 (JCA2007))

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We present a 300MHz surface acoustic wave (SAW) oscillator for gas sensor application. As the oscillator element, SAW delay lines on an ST-X quartz substrate with low insertion loss and single-mode-selection capability were developed. And they were structured by Electrode Width Control Single Phase Unidirectional Transducer (EWC/SPUDT) configuration and comb transducer. The coupling of modes (COM) model was used to predict device performance prior to fabrication. The measured frequency response S_<12> showed a good agreement with simulated results; a low insertion loss of less than 9dB and a linear phase in the 3dB bandwidth were observed. The experimental results show that the baseline noise of the fabricated oscillators was typically up to 〜0.7×10^<-7> in a laboratory environment with temperature control. The oscillator was successfully applied to a gas sensor coated with fluoroalcoholpolysiloxane (SXFA) as the sensor material for O-isopropyl methyphosphonofluoridate (GB) detection, and a superior threshold detection limit was obtained (≪0.4mg/m^3).
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