A new tool for obtaining the frequency characteristics of the interaction of emission sites with adsorbates was developed. The technique is based on the use of a computerized field projector with online processing of patterns of the distribution of emission sites over the cathode surface. The resulting dependences reflect the frequency of fluctuations in the activity of individual sites. The influence of the choice of the threshold brightness parameter, the proximity of the sites to each other and the stability of the total emission current on the shape of these curves is considered. The coincidence of the shapes of curves constructed by various methods (in online mode, in post-processing mode and in the mode of random processes modeling) is analyzed.