When I change the RBW or add input attenuation on my spectrum analyzer, the noise floor changes. Why?
The noise floor is caused by thermal noise generated in the instrument's circuits. When you select a wider resolution bandwidth, more noise comes through the filter and causes the noise floor to rise. When you add an internal input attenuator, modern instruments respond by increasing IF gain to maintain a constant reference level. The increased gain causes the noise floor to rise by an amount equal to the added attenuation.