In an ideal world they would be zero. Typically intensities of less than .5 % are considered ok. One has to distinguish between true spinning side bands that are typically in phase with the main signal and Q-modulation side bands that are out of phase. Q modulation can occur when the sample is out of balance for example because it is a cheap tube or if the cap sits on the tube crooked or anything else which will make the system unbalanced. Clemens