Image cancellation in frequency converters for spectrum analysers

A process for eliminating image rays generated in a frequency converter that generates actual rays and image rays from frequency rays of an input signal, the frequency converter comprising:

• a first mixer that mixes the input signal and a signal provided by a first local frequency source, and
• a filter assembly that filters the mixed signals output by the mixer, the process comprising:
• a first sweep, wherein a frequency of the signal provided by the first local frequency source is varied and wherein a signal level including actual and image rays at an output of the frequency converter is recorded for a set of converted frequencies;
• a second sweep, wherein the frequency of the signal provided by the first local frequency source is varied and wherein the signal level including actual and image rays at the output of the frequency converter is recorded for said set of converted frequencies, said second sweep being carried out so that a frequency of the actual ray generated in the second sweep by an input ray is the same as a 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;
• determining if a ray-including signal level was recorded both during the first sweep and the second sweep for at least one converted frequency;
• and
• displaying an indication of a presence of a ray for said at least one converted frequency on a screen only if said determination is positive,
• wherein the first sweep and the second sweep are performed with respect to a common input signal,
• wherein the filter assembly comprises a first bandpass filter and a second bandpass filter that filter each a respective band of frequencies,
• wherein said first bandpass filter filters the mixed signals, which are outputted by the first mixer during the first sweep and which do not pass though the second bandpass filter,
• and wherein said second bandpass filter filters the mixed signals, which are outputted by the first mixer during the second sweep and which do not pass through the first bandpass filter.