Tail current noise suppression in RF CMOS VCOs

Abstract

This paper presents the experimental results of two different techniques, inductive degeneration and capacitive filtering, for reducing the phase noise in tail-biased RF CMOS voltage-controlled oscillators (VCOs). Both techniques prevent the low-frequency tail current noise from being converted into phase noise. The techniques are applied to two distinct VCO designs, showing that the largest phase noise reduction (up to 6-7 dB at 3-MHz offset frequency from the carrier) is achieved via inductive degeneration. Capacitive filtering, however, also substantially reduces the phase noise at high offset frequencies and may therefore become a valid alternative to inductive degeneration, as discrete capacitors are of more common use than discrete inductors.