  The mathematical model of periodically amplitude and phase modulated
sinusoidal oscillation is studied, and its Fourier spectrum is given
analytically. The Fourier spectrum of the model explains the main features of
the frequency spectrum of RR Lyrae stars showing light curve modulation called
the Blazhko effect: among others the appearance of multiplets, the rapid
decrease of their amplitudes in increasing orders, the asymmetry of the
amplitudes of the side frequency pairs, and the possibility of the occurrence
of frequency doublets instead of triplets in the spectrum. The good agreement
of the results of this mathematical model with observational facts favours
those physical models of the Blazhko effect which explain the phenomenon as a
modulation of the oscillation with the modulation frequency, f_m.
