Abstract: The process distinguishes image rays and actual rays generated in a frequency converting device from frequency rays of an input signal. The frequency converting device comprises at least a mixer mixing the input signal and a signal provided by a local oscillator and a filter assembly filtering the mixed signals output by the mixer. At least one sweep is carried out wherein the frequency of the local oscillator is varied and wherein the signal level at the output of the frequency converting device is recorded for a set of converted frequencies. A further sweep is carried out so that the frequency of the actual ray generated in the further sweep by an input ray is the same as the frequency of the actual ray generated in said at least one sweep by said input ray and the frequency of the image ray generated in said further sweep by any input ray does not correspond to a frequency for which a ray was generated in each of said at least one sweep.