What is the difference between resolution bandwidth and video bandwidth in a spectrum analyzer? How do spectrum analyzer measure spectral energy? Is it avarage power in each RBW?
The bandpass filter in the front end limits how much energy width (in frequency) gets to the power detector by setting the RBW ( and also the center frequency F0).
AFTER the power is detected, you can smooth it out (lowpass filter) with the VBW control.