This work is devoted to the nonlinear analysis of the Costas loop and phase-locked loop with squarer. By using the special asymptotical methods for analysis of high-frequency harmonic and impulse oscillations, integro-differential and differential equations for the Costas loop and phase-locked loop with squarer are derived. The analytical methods for calculation of phase detector characteristics are suggested. For high-frequency clock oscillators new classes of such characteristics are described.