IMAGE CANCELLATION IN FREQUENCY CONVERTERS FOR SPECTRUM ANALYSERS

摘要

The process eliminates image rays generated in a frequency converting device generating actual rays and image rays from frequency rays of an input signal. The frequency converting device comprising a mixer (2) mixing the input signal and a signal provided by a local oscillator (4) and a filter assembly (6) filtering the mixed signals output by the mixer (2). In a first sweep and a second sweep, the frequency of the oscillator is varied and the signal level at the output of the frequency converting device is recorded for a set of converted frequencies. The second sweep is carried out so that the frequency of the actual ray generated in the second sweep by an input ray is the same as the frequency of the actual ray generated in said first sweep by said input ray, and the frequency of the image ray generated in the second sweep by an input ray is not the same as the frequency of the image ray generated in said first sweep by said input ray. Then, the process determines if a ray-including signal level was recorded both during the first sweep and the second sweep for at least one converted frequency and displays an indication of the presence of a ray for said at least one converted frequency on a screen (14) only if said determination is positive.