  We present the results of a time series analysis of the long-term radio
lightcurves of four blazars: 3C 279, 3C 345, 3C 446, and BL Lacertae. We
exploit the data base of the University of Michigan Radio Astronomy Observatory
(UMRAO) monitoring program which provides densely sampled lightcurves spanning
32 years in time in three frequency bands located at 4.8, 8, and 14.5 GHz. Our
sources show mostly flat or inverted (spectral indices -0.5 < alpha < 0)
spectra, in agreement with optically thick emission. All lightcurves show
strong variability on all time scales. Analyzing the time lags between the
lightcurves from different frequency bands, we find that we can distinguish
high-peaking flares and low-peaking flares in accord with the classification of
Valtaoja et al. (1992). The periodograms (temporal power spectra) of the
observed lightcurves are consistent with random-walk powerlaw noise without any
indication of (quasi-)periodic variability. The fact that all four sources
studied are in agreement with being random-walk noise emitters at radio
wavelengths suggests that such behavior is a general property of blazars.
