Why reciprocal procedure works?

Accurate characterization of the calibration standards used for the short-open-load-thru (SOLT) calibration of S-parameter measurements of RF devices is crucial if correct measurements are to be obtained. For most reciprocal devices (i.e. with S₁₂=S₂₁) if the calibration coefficients are inaccurate, the \´calibrated\´ S_21A and S_12A values appear not to be identical over all frequencies. We find that we can use this apparent violation of reciprocity to develop a novel procedure for standard characterization. This procedure ("reciprocal procedure") searches for the optimum set of calibration coefficients for which the reciprocity of the device is obeyed at all frequencies