Heretofore, all precision clocks and oscillators have required one or more ovens to control the temperature of frequency dependent components. Ovens always add significantly to the power budget and limit the upper operating temperature. In the 1970s, Jack Kusters, et al. developed the dual-mode oscillator, which allows one mode to be used to measure temperature [1]. Then the other mode is made as frequency independent with temperature as can reasonably be done. The modes used are the B- and C-modes. With knowledge of the temperature behavior for these two modes, the B-mode measurements can be used to compensate the much smaller frequency dependent changes in the C-mode so that the output is temperature independent [2]. In 1968, David W. Allan developed the time-scale algorithm for the atomic-clock ensemble providing time for the USA at the National Bureau of Standards in Boulder, Colorado. German and Allan have modified that algorithm to operate with a quartz-crystal oscillator ensemble. In addition, they have developed an algorithm that uses the ensemble to determine the temperature coefficients for each of the oscillators (clocks) in the ensemble — called automatic temperature compensation (ATC). The output of the ATC feeds the time-scale ensemble algorithm, which has the property of providing an output better than the best clock, and even the worst clock enhances the performance. With this configuration, flicker-floors of parts in 1012 have been obtained with only a gradual deterioration in stability in the long-term. The ensemble output also compensates for long-term frequency drift. To date, the EQUATE timing stability has demonstrated timing errors of less than 100 microseconds for prediction intervals out to 10 days. The ensemble operates without an oven, with micro-degree temperature measurement precision, has been tested over a temperature range of −40 to +85°C and can reasonably operate at even higher temperatures--. This R&D system is now in its bread-board phase, and efforts are in progress to turn it into a small, low-powered, timing product [3]. This quartz-crystal oscillator timing ensemble is an essential part of the EQUATE technology package, which provides both precise timing as well as six-axis inertial sensing. We have demonstrated excellent frequency stability during large dynamic motion and during large temperature excursions. The whole timing and navigation package should be small and require very little power — well suited for a GPS denied environment.
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