Replacing several contiguous SCPC/ FM carriers by na equivalent band of thermal noise does not lead to the exact intermodulation power spectral density at the output of a nonlinear amplifier. However, this work shows that an appropriate use of the results following from this approach can still lead to a solution for the problem of evaluating intermodulation impairments, since a precise computation of the RF intermodulation power inside the frequency bandwidth of any carrier is indeed performed. Furthermore, if the spectrum shape is taken into account the baseband noise can be found in a proper way. A computer program using this technique for transponders accessed by SCPC/ FM carriers of different levels has been developed. In order to validate the proposed procedure, comparisons have been performed with the results based on another recently developed technique, involving a fast algorithm for counting intermodulation products. Replacing contiguous narrow-band carriers by thermal noise is also useful when carriers with different normalized power spectral densities are accessing the transponder.