Transmitting SAW hydrophone

Abstract

A transmitting hydrophone which remotely senses both absolute pressure and changes in pressure has been developed using a surface‐acoustic‐wave (SAW) oscillator. This oscillator is formed by placing a SAW delay line or resonator in the feedback loop of an amplifier. The pressure applied to the hydrophone is converted to a flexural loading of the piezoelectric plate which alters the surface‐wave velocity on the plate, consequently shifting the oscillator frequency. This frequency modulated signal is easily transmitted to a remote location for signal recovery. The piezoelectric plate thickness and length can be chosen to optimize the sensitivity of the SAW hydrophone. The surface‐wave element of the oscillator is a planar structure; therefore, it is simple and inexpensive to manufacture. Furthermore, the size and power requirements of the SAW hydrophone are small, typically less than 50 cm3 a and 300 mW. This paper presents performance data of a number of SAW hydrophones using SAW oscillators in the 40–100‐MHz range in conjunction with a versatile signal demodulation system. Extensive measurements of sensitivity and frequency response were made for both delay line and resonator hydrophones manufactured on LiNbO3 and S‐T cut quartz substrates and compared with measurements obtained from conventional hydrophones in the identical setup. [Work supported by Naval Electronics Systems Command Contract No. N00039‐78‐C‐0042.]