Agilent Technologies PSA Series Service Manual page 72

Troubleshooting the RF Section (E4440A, E4443A, E4445A)
RF Section Description (E4440A, E4443A, E4445A)
Figure 3-1 Example of Down Conversion
In the Third Converter, one of three possible 321.4 MHz IF signals is
selected: High Band, Low Band, and 321.4 MHz from an optional
321.4 MHz input. The 321.4 MHz is fed to the system variable gain
which is used to establish gain at 50 MHz as well as remove front end
frequency response as the instrument tunes across it's frequency range.
The third mixer down-converts the 321.4 MHz second IF to the
21.4 MHz IF.
72 Chapter 3