The scan model of the modern spectrum analyzer usually have both sweep model and FFT model. Under the circumstance of the narrow RBW, the FFT is faster than the sweep model, while if the RBW is wider, the sweep model is faster. 

When the scan bandwidth is less than the analyze bandwidth of the FFT, the FFT model can measure the transient signal; when the scan bandwidth is over the FFT, if we choose the FFT model, it is possible to lose the useful signal. The spectrum analyzer will be distortion.