The use of monolithic piezoelectric crystals as resonators is well known. As the state of the art developed, the phenomenon of energy trapping was discovered by Wilfrid S. Mortley and was published in an article entitled, "FMQ", which appeared in Wireless World, 0ctober, 1951, Vol. 57, pages 399-403. Still later, the behavior of thickness-shear mode quartz resonators was exp1ained both mathematically and experimentally in terms of lateral standing wave trapped energy modes by William Shockley et al. in an article entitled, "Trapped Energy Mode In Quartz Filter Crystals", which appeared in the Journal of The Acoustical Society of America, April, 1967, Vol. 41, pp. 981-993. Parallel field excitation, also known as lateral excitation, was further disclosed in a publication by R. H. Bechmann entitled, "Improved High-Precision Quartz Oscillators Using Parallel Field Excitation", which appeared in the Proceedings of the I.R.E., Vol. 48, March, 1960 at pp. 367-368 as well as U.S. Pat. No. 3,165,651 issued to the same R. H. Bechmann on Jan. 12, 1965. Still more recently, the construction of a trapped energy mode resonator utilizing parallel field excitation was published by Takehiko Uno in an article entitled, "A LiTaO.sub.3 Monolithic Crystal Filter by Parallel Field Excitation", which appeared in the Transactions of The IECE of Japan, Vol. E61, No. 11, November, 1978, at pp. 915-916. There, two rectangular electrodes were deposited on each plate surface with a thin dielectric film deposited thereon to cover both electrode region and intermediate gap areas. With a high frequency voltage being applied to the electrodes, an elastic wave is excited and the energy of vibration is trapped in a central region of the plate by the mass loading effect of the dielectric film, thereby providing a trapped energy mode resonator which is applicable for operation in the VHF region.
Dielectric coatings, however, have been found to lead to frequency instability with the passage of time (aging), to hysteresis in the frequency-temperature characteristic, and they also contribute to the production of phase noise when the resonator is incorporated in an oscillator. These are thought to be due to the crystal/dielectric interface. The dielectric also degrades the resonator operating quality factor or "Q" because it is not, among other things, elastically matched to the crystal.
Lateral field excitation is also disclosed in a co-pending patent application entitled "Piezoelectric Rezonators Having a Lateral Field Excited SC Cut Quartz Crystal Element, Ser. No. 738,697, filed by A. Warner and assigned to the same assignee as the present invention.
It has also been found that energy trapping can be enhanced by the use of tapered electrodes. This concept was disclosed in a publication entitled, "Influence of Tolerances on Acoustic Bulk Wave Resonators Made of Thin AT-Cut Quartz Crystals", which appeared in publication 25 of the Technical Research Center of Finland, by Bertil Godenhielm, ESPOO, Finland, 1979. The energy trapping phenomenon may be utilized with either a plate-like (plano) piezoelectric element or with a contoured piezoelectric element. The contoured element may have a plano-convex profile or a convex-convex profile. Contoured resonators are discussed, for example, in "Resonator and Device Technology" by John Kusters in "Precision Frequency Control ed. by Eduard Gerber and Arthur Ballato, New York: Academic Press, 1985.
Accordingly, it is an object of the present invention to provide an improvement in piezoelectric crystal resonators.
It is a further object of the invention to provide an improvement in piezoelectric crystal resonators excited by lateral excitation.
It is another object of the invention to provide a reduction of frequency aging in piezoelectric resonators.
It is yet another object of the invention to reduce hysteresis in the frequency-temperature characteristic of piezoelectric resonators.
It is yet a further object of the invention to provide an improvement in the phase noise spectrum of a piezoelectric resonator using lateral excitation.
And still a further object of the invention is to provide an increase in the resonator quality factor of piezoelectric resonators using lateral excitation.