A Stable Loss Control Feedback Loop for VCO Amplitude Tuning

In practical implementations of LC oscillators in which the quality factor of the tank is dominated by the quality factor of the inductor, due to dependence of the oscillation amplitude on the square of the oscillation frequency and the bias current of the LC tank, a stable amplitude control loop is essential to maintain a constant oscillation amplitude over the tuning range of the voltage-controlled oscillator (VCO) and to optimally bias the VCO over different conditions. In this paper, an enhanced loss control scheme incorporating an integral feedback to automatically tune the oscillation amplitude of LC oscillators is proposed. The proposed loss control feedback (LCF) loop is conditionally stable with an easy stability requirement to meet and its stability is examined: 1) by linearizing the system around the stable point using a perturbation method; and 2) by numerically solving the nonlinear differential equation of the LCF loop describing the transient behavior of the step response of the loop. A prototype including the LC VCO and the proposed LCF loop has been implemented in TSMC 0.35-mum CMOS process and occupies an area of 0.057 mm² and consumes 8.1 mA from a 2.8-V power supply. The LCF loop, with respect to the VCO, has an overhead of 25% on the area and consumes only 1.3% of the total power. Measurement results of the proposed LCF loop show an 11 dBm amplitude tuning range from -16 dBm to -5 dBm at any frequency over the 2-2.5-GHz tuning range of the VCO. The step response of the loop has a settling time less than 0.5 ns