The authors analyse sources of false Doppler velocity signals of high frequencies (10 mHz and higher) in observations of filaments. In ground-based observations, spectrograph noise and image shifting at the spectrograph entrance slit are the main causes of the false signal. It is shown that using differential methods and telluric lines as reference lines significantly reduces the influence of the first factor. Periodical image shifting along the spectrograph slit can be compensated for during data reduction. In some cases detected high-frequency oscillations appear to be real.