The NxN technique is an application for measuring non-separable devices. NxN is used when devices can only be measured in pairs and information about the individual devices is required. Examples of NxN include:
• Mixers (where one of each pair is an up converter and one is a down converter so that the VNA sees no frequency translation)
• Long cable runs (where two cables are needed to complete a round trip to the VNA)
• Fixture parts (where an individual part cannot be easily measured since one of its interfaces is non-coaxial/non-waveguide).