Class-C VCO With Amplitude Feedback Loop for Robust Start-Up and Enhanced Oscillation Swing

We propose a feedback class-C voltage-controlled oscillator (VCO) that has robust start-up and a large oscillation amplitude. It initially starts oscillating as a conventional cross-coupled LC-VCO for robust start-up and subsequently transforms automatically into an amplitude-enhanced class-C VCO when it reaches steady-state to give improved noise performance. Detailed analysis of the start-up conditions, enhanced oscillation swing, and amplitude stability provides valuable insight into oscillator design considerations. The proposed VCO is implemented in a 0.18-μm CMOS process. The measured phase noise at room temperature is - 125 dBc/Hz at 1 MHz offset with a power dissipation of 3.4 mW at an oscillation frequency of 4.84 GHz. The figure-of-merit is -193 dBc/Hz.