Ratio computed for T = 150 K as a functionof ν_cent for the bandpasses of the radiometers 18S (thick red line) and 27S (thick green line), offseted in frequency, and for a set oftop-hat bandpasses of different width (black thin lines). The vertical dashed lines indicates the central frequencies of the radiometers. The bandwidth of the bandpasses are expressed as function of the effective bandwidth q defined in Eq. (A.21). The q-values for the top-hat bandpasses are printed at the left of each curve, while the Planck/LFI bandpass is 6.5 GHz for 18S and 2.4 GHz for 27S.