B. Capelle

Bulk wave propagation and energy trapping in the new thermally compensated materials with trigonal symmetry

In this communication, we compare the properties of the new piezo-electric materials that belong to crystal class 32: the aluminium phosphate, the gallium phosphate and langasite with those of quartz. In a first part, a comparison of the main piezo-electric properties of these crystals is made. Computed and experimental results concerning the Y rotated cuts and particularly the compensated cuts are given. The most important facts observed are the increase of the coupling coefficient from quartz, to berlinite, langasite and gallium phosphate, and the simultaneous shift of the angular position of the most interesting compensated cuts towards the angles of maximum coupling coefficient. New determinations of the angular positions of the corresponding compensated cuts of gallium phosphate and doped langasite are also reported; the former one displays the largest coupling coefficient and an outstanding thermal stability. In the second part, the energy trapping properties of these four materials are compared. It is concluded that filters having more than 4 times the bandwidths obtained with quartz can be made using the new materials. Extremely high Q factors and the ability to withstand large excitation levels were observed in langasite resonators making use of energy trapping by the geometry (plano-convex). They appear very promising for low phase noise applications. The materials were found to possess an unique set of complementary properties, a high crystalline perfection, superior thermal behaviours, and different coupling coefficients adapted to an extremely broad range of filtering and frequency generation applications.<<ETX>>

Study of gallium phosphate and langasite crystals and resonators by X-ray topography

The gallium phosphate (GaPO/sub 4/) is, as berlinite, a close crystallographic analogue of quartz with the advantage that it has no /spl alpha/-/spl beta/ phase transition. It was shown that this material has very attractive properties for the applications to piezoelectric devices, due to its large coupling coefficients and to the existence of compensated cuts. Furthermore, it can be used up to extremely high temperatures (900/spl deg/C). To obtain much larger crystals, epitaxial growth of GaPO/sub 4/ on berlinite plates with several orientations was performed. The langasite crystals (LGS: La/sub 3/Ga/sub 5/SiO/sub 14/) were obtained by the Czochralski method. This new material has probably one compensated cut with a large coupling coefficient and very reduced angular sensitivity. Using the synchrotron radiation delivered by the DCI storage ring at the LURE (Orsay, France), we have studied by the X-ray topography technique the crystalline perfection of new GaPO/sub 4/ samples and different langasite crystals of different sources. For this study we have used traverse and section topography.<<ETX>>

Air-gap determination of the cut-off frequency of unelectroded plates and effects of local thickness modifications in plane resonators

A method is proposed for obtaining precise measurements of the cutoff frequencies of unelectroded plates. These quantities can then be used in a model, similar to that which has permitted the extraction of the cutoff frequency, to determine the other parameters of the resonator (electrode geometry and mass loading). Two successive applications of these models permit one to remove most of the uncertainties relative to the material constants and to obtain precise values of the equivalent scheme and, if necessary, a response free from anharmonic modes. A model of the resonators with embedded electrodes proposed by T.J. Lukaszek (1971) has been constructed and the properties of this type of resonators computed. The models made for the three types of resonators described here are based on the approximate equations governing the thickness vibrations of piezoelectric plates established by H.F. Tiersten and coworkers (1979). Two methods of resolution of these equations were used: a semialgebraical one and the finite-elements method.<<ETX>>

Berlinite: Characterization of Crystals with a Low Water Concentration and Design of Bulk Wave Resonators

Several modifications to crystal growth methods were successfully investigated to decrease the water concentration in crystals.

Studies of Quartz Resonators by Stroboscopic Topography

Summary Stroboscopic X-ray topography have been carried out to observe the states of acoustic vibrations in quartz resonators. These experiments have been done using the storage ring of D.C.I. (Orsay, France) as a pulsed light source. The studied AT resonators have shown that the vibration mode is directly related to the density and the distribution of the defects.

Optimization of AT berlinite and quartz thickness shear devices for VHF filter applications

Systematic investigations concerning the design of VHF berlinite and quartz resonators were made to find the best designs for the elements of filters for the intermediate frequency of the future pan-European digital radiotelephone of the GSM system. An investigation of the influence of the parameters of the plane fundamental resonators using the AT cut of berlinite and quartz on the elements of the equivalent scheme and on the anharmonic spectrum was made. This was done using a numerical model based on the theory of the essentially thickness modes. Several families of designs were found to lead to nearly satisfactory sensitivities and values of the equivalent scheme, but when a constraint was put on the impedance level and on the sensitivities to the variations of several parameters that are difficult or costly to totally control in an industrial process, one family of designs, using elliptical electrodes with an axis ratio respecting the lateral anisotropy, was found to be much better.<<ETX>>

An analysis of the SAW displacements in quartz and lithium niobate by X-ray topography

In this report we develop investigations which have been made to study the energy distribution of different type of surface acoustic waves in different devices using quartz and lithium niobate. The observations were performed with X-ray topographs obtained by the use of the synchrotron radiation. The time structure of the X-rays has permitted to obtain stroboscopic observations of the amplitude and phase distributions of the Rayleigh waves across the devices. Bragg and Laue patterns were obtained in order to analyse the components of the displacements. The most important results concern the interactions between the dislocations and the SAW.<<ETX>>

New designs for resonators and filters using lithium tantalate

The results of investigations made into improving the characteristics of fundamental and overtone mode resonators are presented. Using the stroboscopic topography technique, it is shown that the rather poor Q factors of the fundamental thickness shear resonators are due to non-stationary plate modes coupled to the fast shear. The use of bar shaped thickness shear resonators permits higher Q factors to be obtained. To obtain overtone resonators with a zero first order temperature coefficient, (TC) doubly rotated cuts, the compensation of the TC of the overtones of the X cut by a layer of silica, and the use of the lateral field excitation are considered. This latter type of resonator is very well adapted to lithium tantalate and permits, using the plano-convex geometry, very large Q factors for overtones with very elevated ranks to be obtained.<<ETX>>

Assessment of material perfection and observation of vibration modes in lithium tantalate by X-ray topography

The crystalline perfection of LiTaO/sub 3/ samples was examined using synchrotron radiation X-ray topography. The two thickness shear modes existing in X-cut plates were observed using either conventional or stroboscopic topography. The fundamental fast shear mode and its anharmonics display strong trapping properties. The slow shear mode is not strongly trapped and extends far from the electroded region. In most cases, the fast shear mode and its anharmonics are coupled to plate modes. Using the time structure of the radiation of the synchrotron, it is shown that, in most instances, these coupled modes include progressive components with a large standing wave ratio. The existence of these progressive waves, characterized by a large amplitude, was observed in all the samples.<<ETX>>

Mode shape analysis techniques using synchrotron X-ray topography

X-ray topography using synchrotron radiation is used to investigate vibrational states in quartz resonators. After a brief review of different methods and possibilities obtained with synchrotron radiation, some observations on vibration modes, especially on coupled piezo-electric components in quartz AT and BT resonators, are reported. The results from experiments reveal time-progressive components and several complex coupled components in AT and BT cuts.<<ETX>>