Design Considerations for High-Frequency Crystal Oscillators

Crystal oscillators are commonly used to generate system clock frequencies in various electronic applications. It is well known that high-frequency crystal oscillators above 30 MHz usually employ series-resonance mode crystals in various configurations with a tank circuit for overtone selection. It is the purpose of this paper to discuss the effects of the crystal shunt capacitance and series resistance on the performance of a large class of series-mode oscillators. Specifically, it will be shown that when the parasitic capacitance across the crystal significantly modifies the transconductance of the amplifying stage, the circuits can become susceptible to a parasitic second mode of oscillation above the series-resonance frequency of the crystal. Limitations on the range of loop-gain values that enables oscillations to start only at the desired frequency will be discussed and simple design equations for a reliable oscillator start-up will be derived. © 1991, IEEE.