Hi Paul,
Are you referring to the spectrum plot in zero-span mode? or the time plot?
In both plots, the input power setting will play a large factor in the noise floor. The BB60C has maximum sensitivity at -50dBm input power/reference level. You want to try to set your input power setting to ~5dB above your expected input level to achieve the best dynamic range.
In the spectrum plot, the noise floor is also affected by the RBW. You can lower the RBW in that plot, but you might have to increase the length of the captures to achieve lower RBWs.
In the time domain plot, the noise of the AM samples will be affected by the IF bandwidth setting. You can expect the time domain samples to decrease by 3dB every time you halve the IF bandwidth.
Let me know if you have additional questions.
Regards
