A low-profile high-performance crystal oscillator for timekeeping applications

A crystal oscillator is described that uses various unusual techniques to achieve double-oven class temperature stability with a single oven. Stability of better than 1 part in 10¹¹ over a temperature range of -40° to +85° C has been demonstrated. The use of the single stage oven allows for a form factor with a relatively low height (19 mm. or 3/4 inch) for an oscillator of this performance class. The low profile facilitates card-based designs. In timekeeping applications such as wireless and telecom synchronization, the temperature coefficient of frequency (tempco) is more important than ever. The temperature extremes involved in wireless base stations increase the error due to temperature while the use of GPS timing receivers to discipline quartz oscillators greatly reduces the error due to aging, leaving tempco as the major component. Numerous design innovations are described such as a hermetic oven mass assembly and a non-ovenized digital temperature controller. This controller permits automated optimization of the oven set point and thermal gain. An overview of a novel zero-gradient oven technique used to achieve the double-oven performance is given, with more details in a related paper. Frequency pulling due to the oscillator circuit components is greatly reduced by a novel balanced-bridge controlled oscillator circuit that is described briefly here and in more detail in another related paper