  We applied the method of continuous wavelet-transform to high-quality
time-frequency analysis to the sets of observations of relative sunspot
numbers. Wavelet analysis of these data reveals the following pattern: at the
same time there are several activity cycles whose periods vary widely from the
quasi biennial up to the centennial period. These relatively low-frequency
periodic variations of the solar activity gradually change the values of
periods of different cycles in time. This phenomenon can be observed in every
cycle of activity.
