A voltage controlled ring oscillator plays very important role in high speed systems due to its easy integration capability. The analysis of different VCOs and their applications are discussed and analyzed in this book. There are number of applications of ring VCOs, one of the most important application is phase locked loop. The main task of the PLL is to maintain the coherence between output signal and reference signal by phase comparison. The proposed PLL using improved performance voltage controlled ring oscillator shows better performance in terms of frequency range and power consumption.