Keysight Technologies N9040B UXA Manual page 33

Making Noise Figure Measurements
Making Frequency Converter Measurements
The bandwidth of the SSB filter limits the maximum frequency range over
which a measurement can be swept. Therefore, SSB measurements are not
suited to very wideband DUTs.
Filtering is needed to select the wanted sideband. A swept noise figure
measurement is then possible even if the LO cannot be swept.
N9069C Noise Figure Measurement Guide 33