Abstract: A method for distinguishing between real and spurious responses in a swept frequency spectrum analyzer in a convenient and on-going way uses sweep data from two different, but equivalent, paths through the spectrum analyzer front end. The setup of the spectrum analyzer front end continuously alternates between the first and second of these setups. A sweep of the spectrum using the first setup is digitized and stored, as well as displayed. The sweep obtained using the second setup is also digitized and stored, as well as displayed. However, at the same time that the current sweep data is being displayed, the stored sweep data from the previous sweep using the other setup is also displayed at the same location on the screen. At anyone time, there are two displays on the screen, overlaid with each other as exactly as possible.