Phase detector characteristics are computed in synchronization systems. To construct an adequate nonlinear mathematical model of phase locked loops (PLL), the characteristic of a phase detector (PD), which is a nonlinear element whose input is fed with signals from a reference oscillator and a voltage controlled oscillator (VCO) and whose output contains a low-frequency correcting signal, is determined. The high-frequency properties of the oscillators are formulated and computing the characteristic of a phase detector are computed.