Performing conducted emissions measurements on today's products usually involves testing switching-mode power supplies. Even though switchers are light weight and economical to use, they do produce broadband noise which can couple on to power lines. This broadband noise can cause overloading of the receiver's or analyzer's first mixer producing measurement errors.
With the use of a preselector, the bandwidth of the energy presented to the first mixer is reduced, thereby reducing mixer overload and measurement errors.
The Keysight 11960A EMC preselector is a series of switched band pass filters designed to reduce the bandwidth of the energy present at the first mixer of the EMC analyzer.