Comparison between the original signals represented by the polynomial interpolations in log–log of the ESMB without any error or adding relative errors and the corresponding filtered ones. We display the signal according to the original tabulation which is equally spaced in log ν, and multiply the antenna temperature by a certain power of the frequency to better visualise small-scale effects due to the added noise superimposed to the general trend. We consider a narrow frequency interval corresponding to nine grid points, as in those adopted in the differential scheme to numerically compute subsequent derivatives (Sect. 5). See legend and text.