Quartz Crystal Oscillators with Direct Resonator Heating

This paper describes a newly patented, low power, ovenized oscillator with the crystal heated directly by means of a thin film resistive heater deposited on its surface. Mounting the resonator inside an evacuated enclosure forms a miniature "thermos" bottle. The only significant path for heat loss is through the resonator mounting ribbons and posts and these, being long and thin, have quite high thermal resistance. Pulse duration modulation controls resonator heating current. Phase noise, Allan variance, and aging all appear unaffected by direct resonator heating. Practical results for an oscillator employing these concepts are encouraging. Frequency-temperature stability better than ± 2.5 x 10-8 from 0 to 70°C has been achieved using a third overtone 10.230 MHz SC-cut resonator with room temperature power consumption of about 300 mW. Theoretical analysis shows substantial heating power reduction is possible by introducing additional thermal resistance in the form of a glass ring as part of the resonator mounting structure. Room temperature power consumption under 100 mW appears feasible.