Shifting the input spectral densities with  ± Δν_LO =  ± 10   MHz and multiplying by the (IF) bandpass shape (see Fig. 2) returns the “measured” spectra P_sig,ref. The upper panel shows the results for the simpler case, while the lower panel includes a standing wave contribution (ν_sw = 50   MHz). Note that for the method described in Sect. 4.4 we used a smaller standing wave frequency (ν_sw = ν_LO = 10   MHz) in contrast to the other examples in order to avoid resonances (see also Sect. 4.4.1).