Keysight N9912A Service Manual page 63

Theory of Operation
Operation in Modes
Figure 5-5 Signal Paths for Two Port CAT/NA Measurements
The RF IN port is used as the second port.
Two sweeps are required for a measurement: one sweep for the NA-R signal
and a second sweep for the SA signal. Because of the shared LO and A/D, no
phase relationship is available, only magnitude.
When measuring the SA signal, the source and SA receiver step through each
frequency point in the sweep. (The SA receiver is effectively in step mode and
tuned to the source frequency.)
The SA Pre-amp is always switched out.
The RF In Atten is not under user control. For a given frequency, the ADC
Overload Adjustment determines the setting for this attenuator to prevent
over-range warnings.
Keysight N9912A Service Guide 5-23