Tail Current Noise Suppression in RF CMOS VCOs

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 or more common use than discrete inductors.