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When to use Port Extensions vs Electrical Delay on the VNA.

Electrical delay causes the VNA's phase data to be mathematically shifted as though it were sent through a lossless delay line.  It is useful to "unwind" a phase measurement or smith chart measurement and look at the data's deviation from a linear phase response.  (The Marker -> Delay function is a quick way to set the electrical delay.)   Port Extension is another mathematical adjustment that effects the data as though the measurement test ports were extended by a certain amount.   It is commonly used to remove the delay of adapters or a fixture that is attached to the test port cables after a coaxial calibration is performed.

As an example, suppose that the port 1 extension is set to 10 nsec, and the port 2 extension is 12 nsec.  This would cause the measured data to be adjusted with the following delays: S11, 20 nsec; S21, 22nsec; S12, 22 nsec; and S22, 24 nsec.  On the other hand, if the electrical delay is set to 15 nsec, then the data for any measurement will be adjusted by the same amount, 15 nsec.

For both techniques, the measurement data is always adjusted by the time delay value.  However, the VNA also gives a display of the delay in units of distance, for convenience.  If you know the relative velocity of the cable, adapter, or fixture you are using, you can enter the number and make your distance reading more accurate.


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