Fixture compensation is an automated length offset and loss compensation for test fixtures with up to four ports (for 4-port analyzers). The analyzer performs a one-port reflection measurement at each port, assuming the inner contacts of the test fixtures to be terminated with an open or short circuit.

Fixture compensation complements a previous system error correction and replaces a possible manual length offset and loss correction. For maximum accuracy, it is recommendable to place the reference plane as close as possible towards the outer test fixture connectors using a full n-port calibration. The fixture compensation is then carried out in a second step, it only has to compensate for the effect of the test fixture connections.

The following features can further improve the accuracy of the fixture compensation:

• Direct Compensation provides a frequency-dependent transmission factor (instead of a global electrical length and loss).

• Open and Short causes the analyzer to calculate the correction data from two subsequent sweeps. The results are averaged in order to compensate for errors due to non-ideal terminations.  

Auto Length and Loss compensation is a descriptive correction type: The effects of the test fixture connection are traced back to quantities that are commonly used to characterize transmission lines. Use this correction type if your test fixture connections have suitable properties in the considered frequency range:

• The electrical length is approximately constant.

• The loss varies essentially due to the skin effect.

Refer to section Channel - Offset for details.

Direct Compensation provides a frequency-dependent transmission factor. The phase of the transmission factor is calculated from the square root of the measured reflection factor, assuming a reciprocal test fixture. The sign ambiguity of this calculated transmission factor is resolved by a comparison with the phase obtained in an Auto Length calculation. This compensation type is recommended for test fixture connections that do not have the properties described above.

A Direct Compensation resets the offset parameters to zero. In the Electrical Length, Mechanical Length, and Delay dialogs, a checkmark indicates the ports for which a direct compensation has been carried out.  

A non-ideal open or short termination of the test fixture connections during fixture compensation impairs subsequent measurements, causing an artificial ripple in the measured reflection factor of the DUT. If you observe this effect, an Open and Short compensation may improve the accuracy.

Open and Short compensation is more time-consuming because it requires two consecutive fixture compensation sweeps for each port, the first with an open, the second with a short circuit. The analyzer automatically calculates suitable averages from both fixture compensation sweeps in order to compensate for the inaccuracies of the individual Open and Short compensations.