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 PNA models.
To view the options that are installed on your PNA, click Help then About Network Analyzer
Frequency Offset (Option 080) - Allows basic frequency converter measurements
Frequency Converter Application (Option 083) - Allows you to easily make extremely accurate scalar and phase frequency converter measurements.
Tutorials - Learn how to make specific Frequency Conversion Device Measurements.