The applicability of poly(vinylidene fluoride) (PVDF) thin stretched films to the excitation of surface acoustic waves (SAWs) is discussed. Previous results with grating transducers reveal high losses, which are attributed to large attenuation in the polymer. An experimental study was carried out on a LiNbO3 YZ delay line with the input transducer in the form of an interdigital PVDF pair. Untuned insertion losses of 80 dB were obtained at room temperature and frequency of 43 MHz. We demonstrate that these losses are largely caused by the high shear compliance of the film, which leads to degradation of the transducer electromechanical coupling. Employing the piezoelectrically strong thickness-longitudinal mode of the film, a 40 dB loss reduction was achieved. To take full advantage of this mode, a split-electrode grating transducer model with alternating polarity of electrodes is discussed. This is expected to allow efficient SAW excitation at frequencies up to tens of MHz.

Efficiency of poly(vinylidene fluoride) thin films for excitation of surface acoustic waves

The whole acoustic field emanating from a metallic interdigital transducer (IDT) deposited on an anisotropic half-space is computed. Extending the classical Lamb approach, it is shown that the field can be expressed by an integral whose singularities give rise to both Rayleigh surfaces waves (SAWs) and well collimated bulk waves (BAWs). For a Y-cut, Z-propagation quartz crystal a horizontal shear wave is excited; it is shown that the orientation and width of the main lobe can be chosen by changing the frequency.

Bulk waves radiated from an interdigital metallic transducer deposited on an anisotropic substrate

Acoustic power flow in a LiNbO3 single crystal was calculated to find the optimal crystal cutting orientation and the wave propagation direction for the efficient excitation of acoustic waves in the crystal. By solving the stiffened Christoffel equations and calculating the acoustic Poynting vectors with piezoelectric effect included, maximum longitudinal acoustic power flow in a 30° Z‐rotated, Y‐cut LiNbO3 crystal was found.

Optimal crystal cuts and propagation directions for excitation of acoustic waves in LiNbO3

http://ieeexplore.ieee.org/iel5/10285/32718/01535465.pdf

On the Reflection of Bulk Acoustic Waves in Rotated Y-Cut Quartz

The various relations for bulk acoustic wave (BAW) propagation in materials of arbitrary anisotropy and piezoelectricity have been expressed in forms suitable for an interactive graphics presentation of the wave properties. The resulting menu-type program a1 lows the simultaneous display of curves in polar or Cartesian coordinates of slowness or velocity, power-flow angle, and electromechanical coupling constant for propagation in an arbitrary plane. Numerical data for propagation in a particular direction in such a plane is displayed when a line cursor is moved to the desired direction. The computational part of the program generates a numerical data file containing all the calculated BAW properties for an arbitrarily selected crystal plane. The interactive part of the program allows the user a variety of choices for displaying simultaneously up to four seperate panels for data and curves pertaining to the selected plane.

Interactive Computer-Aided Analysis of Bulk Acoustic Waves Using a Personal Computer

Absrruct-The performance of a conventional pair of surface acoustic wave interdigital transducers (IDT’s) used to generate bulk acoustic waves is analyzed. An IDT located on the upper surface of a thick, rotated Ycut quartz crystal acts as the acoustic equivalent of an array antenna, launchingacollimated beam into the crystal bulk. The beam is reflected off the crystal bottom and redirected towards the upper crystal surface where it is intercepted by a second receiving IDT. Closed-form expressions are derived for the important electrical and acoustical delay line characteristics and comparison is made with experimental measurements. The analyzed characteristics of the reflected bulk wave (RBW) delay line are significantly different from either the SAW or SSBWand include a) a frequency bandwidth directly proportional to transducer length, b) a center frequency determined not by finger-to-finger spacing but rather by the ratio of transducer separation to crystal thickness, c) a nonresonant radiation resistance, d) the capability to synthesize filter designs in which the frequency response is proportional to the correlation of the individual transducer finger weightings, and e) the capability to modify temperature stability by change of transducer separation and crystal thickness. ULK ACOUSTIC wave radiation generated by interdigital B transducers has historically been considered as spurious, often degrading or interfering with the performance of devices based upon surface acoustic wave (SAW) excitation. Within the last several years, however, there has been increased interest in utilizing the bulk waves generated by IDT’s for practical microwave applications. Such surface-generated bulk waves make use of the IDT as an acoustic equivalent of a phasedarray antenna, with the electrode fingers constituting the array elements. The phasing between these elements is controlled by the applied signal frequency thereby allowing the direction of the acoustic beam radiated into the bulk to be scanned.

M.A. Goodberlet

Papers

The Excitation and Detection of Surface-Generated Bulk Waves