Abstract
The cause of abrupt frequency jumps observed during the aging of high-stability quartz oscillators is investigated. For this purpose a precise method of measuring the resonant frequency versus drive current is used, and this method is applied to AT-cut crystal resonators used in oven controlled crystal oscillators (OCXOs). Experimental results show that there are some cases where the resonance frequency jumps when crystal current is changed; this phenomenon originates with spurious resonances near the fundamental mode, and the current that causes coupling between the spurious and fundamental modes shifts according to crystal driving time. Furthermore, we show that the abrupt jumping in long-term frequency observation is caused by a "shift" of the current that causes the couplings.
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