dual conversion receivers:
The reason for using dual conversion is that it narrows the needed band with. When you have a single tuned crystal receiver it uses one I.F. frequency lets say it’s 10.7mz that means it will only allow the 10.7mz frequency through the crystal lattice filter but it is not perfect there is a (+) and (-) tolerance. Each time you pass the frequency through a I.F the tolerance is smaller allowing more useable frequency’s for a given band with it also cuts down on outside inference.