Jitter
Period of ring oscillator vibrates in a random manner T=T+T' where T' is a random value. In high-quality circuits range of T' is relatively small compared to T. This variation in oscillator period is called jitter. Local temperature effects cause the period of a ring oscillator to wander above and below the long-term average period: When the local silicon is cold, the propagation delay is slightly shorter, causing the ring oscillator to run at a slightly higher frequency, which eventually raises the local temperature. When the local silicon is hot, the propagation delay is slightly longer, causing the ring oscillator to run at a slightly lower frequency, which eventually lowers the local temperature.
Read more about this topic: Ring Oscillator