State of the art—Quartz crystal units and oscillators

The paper discusses progress made in the field of quartz crystal units and quartz crystal controlled oscillators over the past few years. The field is reviewed in general, but several accomplishments which are thought to be of special importance, are discussed in detail. These subjects include, among others, quartz vibrator characteristics and enclosures, modes of motion including the "trapped energy" concept and long-term drift (aging) of crystal units. The characteristics of various types of oscillators are reviewed including temperature compensated and high precision types, and the problem of short-term stability of crystal controlled oscillators is discussed. Precision oscillators are available today with a daily drift rate as low as a few parts in 10¹¹and a short time stability better than a few parts in 10¹⁰for a time period of one millisecond.