A common question related to calibrations is how often the calibration must be redone. The frequency of re-calibration depends on the environment (both in terms of temperature stability and in terms of the cable/fixture construct that is being used). A calibration lifetime is often limited by the stability of the test port cables through drift or motion. Drift defects directly affect transmission tracking and load match, so if those terms could be easily refreshed, the time before the entire calibration would have to be redone could be extended. This is the concept of a Thru Update.
The idea is to connect just a thru line and quickly refresh the transmission tracking and load match terms without great effort. A Thru Update is a one-step calibration that can be used to refresh a current full 2-port or 1 path 2-port calibration.
Like the thru step in other calibrations, the length of the line being used as well as its loss can be specified to ensure minimal disruption of the reference planes (see Figure: THRU UPDATE Setup Menu and THRU INFO Dialog). The entry methods for these parameters are the same as in the normal calibration procedures.
THRU UPDATE Setup Menu and THRU INFO Dialog
1. Thru Line selected allows user entries for length, line impedance, line loss and frequency.
2. S2P Thru selected provides buttons for loading, viewing, and characterization (to generate S2P files).
Also, as with the manual calibrations, a .s2p file describing the thru S-parameters can also be loaded and will be used during the update. Depending on the calibration, the reflection parameters in the .s2p file may or may not be used ('not' for a transmission frequency response calibration). A characterize thru option is available to calculate the thru's .s2p file using two previously stored reflection calibrations (one with and one without the thru present).
The thru path of an AutoCal module (see AutoCal Calibration of this guide) can also be used for thru update and this can be useful at times if an adapter is not readily available and the AutoCal module has an appropriate connector combination. Only reciprocal processing is available when using this path for thru update as the loss levels are non-negligible.
