Response of ring oscillator to periodic interference on the power supply

Periodic noise is one of the most common interference sources in the power distribution network of integrated circuits. It is well known that this noise can deleteriously affect timing margins and limit the circuit performance. This paper presents a theory on the response of ring oscillators in presence of periodic interference on the power supply. The theory links the cycle time of ring oscillators to the probability density function of the interference. Various waveforms of interference are studied, such as sinusoidal, triangular, and square. In addition, symmetrical and asymmetrical interference waveforms are also considered. Both transistor level HSPICE simulations and real measurements are performed to validate the theory. The obtained results are suitable with the model over a wide range of interference frequency. The valid range of the model, multi-tone interference, and methods to reduce the sensitivity of ring oscillators are also discussed.