A. Ronnekleiv

Diffraction in SAW Multistrip Beam Compressors

SAW multistrip couplers provide an efficient way of compressing wide aperture acoustic beams into narrow beams. Laser probe measurements have verified that diffraction in the multistrip coupler contributes significantly to the resulting beam profile. The paper presents a model for diffraction in multistrip beam compressors. Numerical calculations on a nominally 15:l compressor are compared with laser probe measurements. The measurements confirm the calculations in all essential details.

Amplitude and phase compensation of RAC-type chirp lines on quartz

A novel method for correction of the amplitude and phase response of RAC (reflecting array pulse compressor)-type chirp lines on quartz is demonstrated. The corrections are obtained through normal or reactive RF sputter etching of the lines through a moving slit. For amplitude correction the material selectivity of reactive sputter etching is used to correct the groove depth after initial fabrication of the lines. A thin Cr film, left in place in the finished line, is used as an etching mask. Phase correction is done by sputtering off gold from a gold line between the two arrays. Using this method a Kaiser-Bessel weighted line with a chirp rate of 0.0576 MHz/ mu s and 120- mu s dispersive delay was made, and weighted RMS (root mean square) amplitude and phase errors of 0.086 dB and 0.45 degrees were obtained.<<ETX>>

A fast and accurate solution of the one-dimensional charge distribution in a non-periodic structure

Several SPUDT structures proposed in recent years involve transducer electrodes with wide fingers compared to the smallest finger gap. For these structures the charge distribution will deviate significantly from the results of the fast method, especially for finger widths comparable to the wavelength. Within the approximations of the quasistatic charge distribution we will present a one-dimensional solution in a non-periodic structure, which can be found to desired precision. A closed form solution for both the coupling to a SAW and the potential for an arbitrary charge distribution, given by a predescribed function modified by a Taylor series, has been found. By using these closed form solutions, the boundary conditions for excitation or interaction with a SAW in a transducer structure are satisfied by solving a matrix equation.

SAW reflections on ST-cut quartz grazing angles

Characteristics of surface acoustic wave (SAW) propagation across straight boundaries between areas of different surface conditions must be known to predict wave propagation in SAW waveguides. Actual waveguides consist of combinations of free and metallized surface, and also areas with metal strips such as in interdigital transducers. In particular, waves will impinge on the discontinuities at low grazing angles in wide waveguide structures, as is often the case in SAW filters. Laser probe measurements on reflection at boundaries between several of these surface types on ST-cut quartz are reported. The measurements have given a new value of the anisotropy parameter b=1+ gamma approximately=1.29 for ST-cut quartz. Using this parameter value the measured results seem to be in general agreement with published theories.<<ETX>>

A reflective array structure using STW to SAW conversion

Experiments with a structure consisting of two nondispersive arrays of reflecting grooves on ST-cut quartz are reported. In this folded structure, surface transverse waves (STWs) propagating orthogonally to the crystal X-axis are converted into surface acoustic waves (SAWs) in the first array by a 90 degrees reflection. The STWs are confined to the surface by mechanical loading of a thin aluminum film, and are transmitted and received by interdigital transducers. Experiments were conducted at 250 MHz using 0.5-mm-wide arrays and transducers with 15 periods. Measured untuned insertion loss for one of the devices is 54 dB. Laser probe measurements of the SAWs converted to or from STWs in the arrays are used to probe STW propagation and STW to SAW coupling in the arrays.<<ETX>>

A theoretical investigation of STW propagation at abrupt or distributed changes in surface loading

Two ways of calculating the STW propagation characteristics for waves along a surface with varying mass loading are presented. One method is best suited for abrupt transitions and is based on conventional eigenmode expansion. In the other the boundary conditions at the surface are expressed as an eigenvalue problem. This method allows arbitrary mass loading profiles to be analyzed. A set of scaling rules is given which allows field distributions and losses obtained for one geometry and surface loading to be used also for scaled structures. The scaling applies in a parabolic approximation of the shear wave slowness curve. Different types of mass transitions are studied, as abrupt, stepped, or graded transitions. For abrupt transitions in the parabolic approximation the losses are, as expected, found to be as given by a simple mode overlap integral, brit with some ringing in amplitude and phase at the surface downstream from the transition. Graded or in other ways distributed transitions may have much reduced losses and ringing