Why, oh why ?
I think every scope I've used expressed FFT amplitude units in dBm in a 50 ohm system.
On my 3000T, I had to look at some signals this morning and was surprised to find amplitude 13.01 dB down. WTF? Of course, they are in dBV, with no option to change.
As a sanity check, I fired up my 52486A and there they were ... dBm.
What were they thinking?