Two-Stage Self-Limiting Series Mode Type Quartz-Crystal Oscillator Exhibiting Improved Short-Term Frequency Stability

A simplified model of the transistor sustaining stage employed in common quartz-crystal oscillators is presented. Examination of the model, including associated noise sources, provides an explanation for general differences observed in the output-frequency spectra of several types of widely used self-limiting crystal oscillator circuits. A self-limiting quartz-crystal oscillator circuit configuration is described that has been specifically designed to exhibit simultaneously each of the three important circuit characteristics necessary for improved oscillator short-term frequency/phase stability: large value of oscillator resonator loaded Q, adequate suppression of 1/f flicker-of-phase type noise, and improvement in oscillator ultimate signal-to-noise ratio. Several models of the oscillator circuit have been constructed employing high quality third overtone 5-MHz AT- and BT-cut quartz resonators. Measurement of oscillator short-term frequency stability using conventional phase lock and sampling techniques confirm attainment of substantial improvement in oscillator short-term frequency stability when compared to conventional self-limiting oscillator circuits.