An Optimum Current Mirror Ratio for Low Phase Noise LC–VCO

A design technique for a low phase noise inductor- capacitor voltage-controlled oscillator (VCO) using an optimum current ratio between the oscillating core and the bias circuit is proposed. Conventionally, it is preferred that the current in the oscillating core is maximized to reduce the phase noise. In this letter, however, we find that an optimum current ratio exists for low phase noise for a given power budget. To find the optimum ratio, a theoretical analysis is performed and verified by a prototype VCO using a 0.13 mum CMOS process. Based on the analysis and the measured data, the optimum ratio is shown to be 2 ~ 3.