As the frequency is increased further, the signal should disappear and then reappear at a higher frequency. The signal at the higher frequency which corresponds to the 87Rb isotope will be weaker since the 87Rb isotope comprises 28% of natural rubidium as compared to 72% for 85Rb. If the two separate resonances cannot be picked out then it may be necessary to reduce the amplitude of the modulating field. Why is this necessary?