Design and Performance of Ultraprecise 2.5-mc Quartz Crystal Units

The quality of a quartz crystal frequency standard is determined by the crystal-controlled oscillator, and particularly by the mechanically vibrating, piezoelectrically excited quartz plate. Special quartz crystal resonators, characterized by high Q, excellent frequency stability under shock and vibration, and small change with time, have been developed for use in a new general-purpose, extremely stable frequency standard. The development of improved oven and oscillator circuits has contributed substantially to this improved standard, and will be reported in a separate article. Over-all performance of an experimental oscillator has been reported briefly, 1 and similar oscillators are in operation at the Xa1193 1194 TIIR BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1900 tional Bureau of Standards (see Section 4.5 of this paper), the Naval Research Laboratory and Bell Telephone Laboratories. In this article particular emphasis will be given to the quartz resonator, considering (a) the design principles, (b) the development of the present design and the related processing techniques and (c) the properties of the quartz resonator as a circuit element, including its thermal, mechanical and temporal characteristics. In order to limit the scope, a cursory knowledge 2,3 of crystal unit fabrication will be assumed, and only a brief recapitulation of facts already published will be given. The development of highly stable crystal resonators is a continuing work, because each new achievement in frequency accuracy and stability generates the need for still greater accuracy and stability; to meet these needs, the underlying causes of frequency aging and many other special aspects of the behavior of crystal-controlled oscillators must be more fully understood.