Frequency Offset measurements require the VNA source to tune to a different frequency than the VNA receiver.
This capability is offered as an option on some VNA models
To view the options that are installed on your VNA, click Help then About Network Analyzer
Frequency Offset (Option S93080A) - Allows basic frequency converter measurements
Frequency Converter Application (Option S93083A/B) - Allows you to easily make extremely accurate scalar and phase frequency converter measurements.
Tutorials - Learn how to make specific Frequency Conversion Device Measurements.