Frequency Shift of Ring Oscillators Due to Radio Frequency Interference on the Supply

It is well known that ring oscillators suffer from jittering due to radio frequency interference on their supply. This paper shows that there is another effect of radio frequency interference on the ring oscillator. Although its average voltage is zero, a sinusoidal interference in radio frequency range causes a nonzero accumulated drift on the oscillation of the ring oscillator. As a consequence, the operation frequency of the ring oscillator is shifted. A theory with accompanying equations is developed to explain that phenomenon. The proposed theory shows that the normalized frequency shift is determined by the amplitude of the interference, and is in a certain range almost independent from the frequency of the disturbance. The relationship between the interference amplitude and the frequency shift is determined by how the inverter delay depends on its dc supply voltage. Both transistor level simulations and measurements are performed to verify the theory. The valid range of the theory is discussed. The effect shown in this paper provides a potential method to detect the intensity of electromagnetic interference over a wide frequency range. It also represents a potential method to characterize the interference propagation path in packaged system, such as integrated circuits.