Phase shift produces measurement error in the test fixture’s transmission line because the wavelength at RF frequency is very short relative to the transmission line’s physical length. To remove this measurement error, you must set the electrical length of the test fixture (electrical length from 7-mm terminal to DUT connection plane).
For the E4991B, electrical length values are individually registered for standard Keysight test fixtures. By selecting the model number of a test fixture, the electrical length is set automatically. When connecting test fixtures custom-made by the user, it is necessary to input the electrical length values.
If you are using a registered fixture, then press Cal > Cal/Compen > Accessory > select the registered fixture. In this example, select 16197A.
If you are using other non-registered fixture, press Cal > Cal/Compen > Accessory > Arbitrary. Then, enter the length of the fixture by pressing Cal > Cal/Compen > Accessory Length > enter the length of the fixture, e.g. 1mm (Press 1 > k/m).
After setting the electrical length, confirm that Arbitrary 1.0mm is shown in the status bar on the bottom of the screen.