An ultra stable quartz crystal oscillator (USO) must obviously be ovenized. Frequency fluctuations partly come from static changes of the ambient temperature as well as dynamic ones. Both effects are detailed and their consequences on the oven design are discussed. The static frequency deviation is mainly related to the efficiency of the thermal regulator ' for example, to achieve a relative frequency fluctuation a few l0~(10) over an ambient temperature range of [-30°C+70°C], the static thermal gain must reach at least 1000. However, a standard Proportional and Integral thermal controller which can eliminate the static error is unable to do this for the fast thermal disturbances. Here the thermal filtering must work in accordance with the cut-off frequency of the frequency-temperature transfer function of the quartz resonator. Various methods for controlling the oscillator temperature are presented : the usual method consists of using more than one temperature-controlled ovens. This is often a volume-consuming process. An alternative approach, which is much simpler, is to add a "light" compensation effect to the feedback control system. Lastly, a third way to improve the temperature regulation is based on a distribution of the monitoring power. Obviously, a mixer of those solutions is possible. Advantages and drawbacks of each of them are discussed. Practical results are shown and illustrated with 10MHz USO devices.
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