Keysight Technologies N5511A User Manual page 174

Residual and Additive Noise Measurement Fundamentals
Frequency translation devices
In the case of a dual channel additive measurement on a frequency translating
device, 3 devices are needed, see Figure 7-7. One of the devices will be the
DUT. Because the LO inputs of the phase detectors must be at the same
frequency as the RF input, two additional devices are needed. These are
inserted in the LO paths for each channel. These translating devices need not
be identical to the DUT, but must have the same translating factor.
Performance of each of the two devices is not critical, as noise contributions
from each will be removed by cross-correlation since they are in separate
channels. The only factor that can be affected is time, as worse performing
devices will impact the amount of cross-correlations required to reach the
noise floor of the DUT.
Figure 7-7 Dual Channel Measurement Setup Example for Frequency Translating Device
174 N5511A Phase Noise Test System User's Guide