I am measuring the noise floor of the scope, I have measured it in two ways:

1. Measure the RMS noise – received ~-40dBm

2. Made an FFT on the noise and measured the level in dBm – received ~85dBm

I expect two vales to be the same, could you point me to my error?

1. Measure the RMS noise – received ~-40dBm

2. Made an FFT on the noise and measured the level in dBm – received ~85dBm

I expect two vales to be the same, could you point me to my error?

On the Infiniium scopes, at least, it's easy to measure the RMS noise, just by adding an AC RMS measurement to an unconnected, but active, channel. The measurement will be in Volts, though, so you'll have to do the conversion. It will be different for each settign of the internal attenuators.

AG