New methods, for the design of different block diagrams of PLL, using the asymphtotic analysis of high-frequency periodic oscillations, are suggested. The PLL description on three levels is made: 1) on the level of electronic realizations; 2) on the level of phase and frequency relations between inputs and outputs in block diagrams; 3) on the level of differential and integro-differential equations. On the base of such description, the block diagram of floating PLL for the elimination of clock skew and that of frequency synthesizer is proposed. The rigorous mathematical formulation of the Costas loop for the clock oscillators are first obtained. The theorem on a PLL global stability is proved.