Impact of AAC design on phase noise performance of VCOs

The automatic amplitude control (AAC) system is a servo-circuit of voltage- controlled oscillators (VCOs) that proves important for the practical imple mentation of fully integrated RF front-ends. Nevertheless, the noise contri buted by the AAC can degrade the phase noise performance of the oscillator. In this paper, the time-varying noise transfer of the peak detector is ana lyzed, since it is recognized as the critical noise source in the simple AA C loop taken as reference. A fast rule to predict the noise spectral densit y at the peak detector output is given that substantially facilitates the e stimate of the phase noise degradation on the VCO output. The analysis has been carried out both theoretically and by simulation; experimental validat ion has also been provided by means of a discrete-components test board. Gu idelines for low-noise design of the peak detector of AAC are finally propo sed.