No abstract

Tuning or varying the resonance frequency of a transducer by adding an additional layer of ceramic, called the control ceramic, is a method for designing a wide bandwidth transducer. By loading the control ceramic with an inductive electrical load the resonance frequency of the whole structure is variable or tunable over a wide range of frequency. Two designs of tunable transducer are discussed in this paper; a two-plate thickness-driven structure with a fundamental frequency of 240 kHz and a sandwich structure with a fundamental frequency of 30 kHz. Tunable ranges of 1.5 and 1.8 oct are obtained, respectively. It is established that the limit of the tunable range for the two-plate transducer is between the fundamental harmonic and the second overtone of the whole structure, which are defined with the control ceramic short circuited. For the sandwich transducer the tunable range is between the fundamental harmonic and the first overtone of the whole structure, which are defined when the control ceramic short circuited. An analysis based on the Mason transmission-line model shows that the fundamental limit of the tunable range is determined by the fundamental resonance frequency and the dynamic coupling coefficient of the drive ceramic and is affected by the presence of any bonds and shims between the ceramics. To obtain a continuously tunable frequency range the position of the drive ceramic must be between the radiating front-end of the structure and the control ceramic.

Ultrasonic properties of bulk TlSr 2 (M 1−x Yx)Cu 2 O 7 (M = Ca, Sr) high temperature superconductors have been measured between 80 and 280 K. Sound velocity measurements show that all samples exhibit an overall stiffening trend with decreasing temperature. The longitudinal sound velocity shows a pronounced slope change and attenuation peak around 200 K. The results were explained in terms of phase transition and distortion of Tl and O atoms associated with local oxygen ordering. The calculated acoustic Debye temperature does not show strong correlation with the transition temperature. The electron-phonon coupling constant was calculated using the weak and strong limit of the BCS theory as well as the van Hove scenario. 4923 Int. J. Mod. Phys. B 2002.16:4923-4933. Downloaded from www.worldscientific.com by WAKE FOREST UNIVERSITY on 02/04/15. For personal use only.

The chirp response of a thickness-drive tunable transducer for wide range time-bandwidth and sweep rates chirp signals is demonstrated experimentally and computationally. The computational evaluation uses recursive digital-filter model based on the z-transform method. The model is limited to simple lossless structure with no front and backing layers. The model and experimental results show that there is no limit on the maximum sweep rate of the chirp signal but practically the limit is determined by the limitation of the circuit that generates the control voltage that simulates a variable electric load.

The elastic properties and ultrasonic attenuation of TlSr 2 (Sr 0.5 Y 0.5 )Cu 2 O 7 have been determined between 80 and 280 K at frequencies in the 10 MHz region. Electrical resistance versus temperature measurement showed that the critical temperature is 88 K. Ultrasonic measurements showed a change in longitudinal velocity gradient and the attenuation peak around 200 K. The results were explained in terms of the phase transition associated with local oxygen ordering. The shear velocity showed an almost monotonic increase between 220 and 80 K and no attenuation peak is observed in this mode.