Failure in Ring Oscillators With Capacitive Load

Abstract

Ring oscillators, with capacitive load intended for frequency tuning, can fail to sustain stable oscillations. Failure depends on the size of the load capacitors and their equivalent series resistance. This paper first describes the failing mechanism from an intuitive perspective. It then develops an analytical model and provides the equations for the minimum load capacitance and the range of equivalent series resistance at which the ring oscillator fails. The dependency on the number of stages of the ring oscillator as well as the trans-conductance and impedance of its gain stages are also examined. Confirmed by simulations, the model shows that the failing region reduces significantly for ring oscillators with 7 or more stages but that it can be a serious concern for 5 and especially 3-stage ring oscillators.