What is RBW and VBW in a Spectrum Analyzer?

In a spectrum analyzer that uses the heterodyne principle, the instrument sweeps a bandpass filter, called the IF filter, across the spectrum of interest and continually measures and displays the power of signals within the bandpass of the filter. The bandwidth of that filter is called the resolution bandwidth, or RBW. Ideally, you would like to set this bandwidth as narrow as possible, as that would give you the finest frequency resolution. The tradeoff is sweep time. The narrower the resolution bandwidth, the longer the sweep time, but if you sent the RBW too wide, you won't see signals that are close to one another.

After the input signal passes through the IF filter, it goes through a detector, which outputs a signal whose amplitude is proportional to the power passing through the IF filter. This output signal then passes through the video filter. The purpose of this filter is to smooth the noise inherent in the input signal. Its bandwidth is called the video bandwidth, or VBW. The narrower the bandwidth, the less noise there is in the output signal, but again the narrower the bandwidth, the longer the sweep time.