A phase locked loop used as a digitally-controlled oscillator for flexible frequency synthesis

An all-digital phase locked loop (ADPLL) that uses a second fractional-N PLL as its digitally-controlled oscillator (DCO) has been studied, prototyped and tested. This technique allows for an effective implementation of a low-bandwidth ADPLL, exploiting the benefits of a digital implementation while avoiding the complexity of designing a DCO and a Time-to-digital converter (TDC). The reuse of a PLL as a DCO provides for easy interfacing, high linearity, zero drift and very high frequency resolution. An overview of the theory, technique, limitations, applications, and results are presented in this paper.