Quartz frequency standards

This paper reviews progress made over the past few years in the field of quartz crystal frequency standards. Several accomplishments which are thought to be of special importance are discussed in detail. These subjects include, among others, quartz vibrator enclosures, modes of motion including the "trapped energy" concept, and studies of quartz material. It has been demonstrated that the Q of synthetic quartz can be improved to a value equivalent to natural quartz. Progress made in the field of long-term stability of precision crystal units is described and the various reasons for aging of such units is discussed. The characteristics of precision crystal oscillators are reviewed, including temperature compensation and the influences of severe environment. The properties of high-precision crystal frequency standards are described with specific emphasis on aging, and the problem of short-term stability of such standards is discussed. Precision crystal standards are available today with a daily drift rate as low as a few parts in 10¹¹and a short-term stability better than a few parts in 10¹⁰for a period of one millisecond.