INTEGRATED CIRCUIT CRYSTAL OSCILLATOR HAVING DIGITAL AUTOMATIC GAIN CONTROL COMPRISING OSCILLATION DETECTION AND AMPLITUDE CONTROL LOOPS

摘要

A crystal oscillator is coupled to a digital automatic gain control (AGC) having oscillation detection and amplitude control loops. The oscillation detection loop may increase the transconductance (gm) of the oscillator transistor until oscillation is detected therefrom. Then the amplitude control loop detects the amplitudes of oscillations from the crystal oscillator, compares these amplitudes to high and low voltage references and generates digital signals to find a critical transconductance (gm) for an oscillator amplifier and control this gm to maintain a constant oscillation waveform amplitude therefrom. An up/down counter defines the servo control loop bandwidth/update-rate according to an update clock rate thereto. Loop stability is achieved when the control loop bandwidth is less than the start-up time required for the oscillation envelope of the crystal oscillator to grow for oscillation. An oscillator failure detector may also be provided.