In this paper an investigation of different filter prototypes and their applicability to digital phase locked loop design is carried out. A novel design technique using the superior filter prototype for the 4th order Digital PLL is also introduced. The optimum choice of each design parameter is considered, while maintaining realisable component values as a priority. Finally the proposed design technique is used to design a 4th order Digital PLL with optimum filter cut-off, stability and lock time. This 4th order design method is an improvement on existing methods that exist in the literature to date, this is verified using simulation of a Digital PLL designed using the proposed technique. Â© 2006 IEEE.