CML ring oscillators: oscillation frequency

In this paper an analytical model of the oscillation frequency of a differential CML ring oscillator is proposed. The model helps to choose the bias current value to optimize speed, and it is useful to estimate the oscillation frequency changes associated with the bias current change and to process tolerances. Moreover, it allows to relate the propagation delay of a gate evaluated in a ring oscillator to more general and realistic cases. The model is sufficiently simple to be used in pencil-and-paper calculations. Spice simulations run on both 6 GHz and a 20 GHz bipolar technologies show that the model results agree with the simulated data, which differ by less than 10% on average. Figure 1. Ring oscillator topology.