A method of digitizing an analog signal by an analog-digital converter Shannon frequency determined

A method of forming a global spectrum (Sg) of an analog signal (A) to be scanned, wherein: - is sampled with a first analog to digital converter (21, 22) determined Shannon frequency (Fs) , the analog signal (a) to obtain an aliasing (Sr), the analog signal (a) not having been previously filtered through an anti-aliasing filter; - subtracting the aliasing (Sr) a basic spectrum (Sb) to obtain a folded spectrum (Sre), base spectrum (Sb) corresponding to a spectrum without aliasing of said analog signal; - calculating an unfolded spectrum (Sd) from the folded spectrum (Sre) based on the Shannon frequency (Fs) of the converter (21, 22); - are concatenated base spectrum (Sb) with the expanded spectrum (Sde) to form the overall spectrum (Sg) of the analog signal (A).