The SA44B definitely has some shortcomings when it comes to EMC testing. It relies on a software image rejection algorithm to get rid of both image responses and its own spurious responses. This algorithm tends to work better on sparse spectrum with narrow RBWs, where the chance of a spur and an image overlapping is low. When there are many signals closely spaced, especially at low frequency where there may be higher frequencies present mixing with harmonics of the LO, the chance of spurious signals overlapping on the primary and image sweep are high.
This is one of the reasons we strongly recommend the BB60C for EMC work. With hardware image rejection and specified spurious performance, you get reliable EMC sweeps.
