A. Mansano, A. Vilas Boas (Brazil), and A. Olmos (USA)
Crystal Oscillator, Hysteresis Control, Fast Start-Up.
A novel, low cost scheme to control and guarantee proper clock generation in crystal oscillator applications is presented. Disturbances like process, voltage, and temperature (PVT) variations as well as deviations in bias currents and leakage can drive crystal differential output signal to a transitory lower or higher than expected amplitude oscillation. Automatic Gain Control (AGC) architectures using comparators with constant hysteresis have serious limitations to eliminate clock spikes and clock malfunctions under those disturbances. This paper describes an Automatic Gain and Hysteresis Control (AGHC) loop that linearly adjusts the hysteresis of the crystal oscillator comparator close to the differential oscillation amplitude to maintain clock signal frequency and duty cycle constants over PVT and under external disturbances. AGHC also provides a fast clock start-up compared to systems based on the classic AGC. Circuit architecture as well as simulation results based on a standard CMOS 0.18um process are presented demonstrating enhanced features and a faster loop stabilization when compared to classic AGC systems.
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