Periodicity analysis of nonuniform time series by means of a "whitening" method is described. The method is applied to a master light curve for the active galaxy 3C 120 for the period 1967-1989. This light curve has a two-component structure, with a long-timescale component with period 15 years and a noise component that resembles white noise. The study of separate intervals of the observational series shows that the amplitude of the 15-year component is time-dependent.