Simple estimators are given for signal deformation at the output of the correlation processing unit in spectrum parts rejection. Using the relationships obtained, one can reasonably choose the rejection parameters int he allowable signal distortions without modeling all the processing section. The results obtained show the high sensitivity of distortions of the mutual correlation function to the spectrum part rejection and prove that this type of distortions is displayed not so much in the peak reduction of the convoluted signal as in the growth of its remains.