The Impact of Choosing Different Reference Impedances on Calibration Results

The reference impedance ($z_{\mathrm{r}\mathrm{e}\mathrm{f})}$ of an S parameter after calibration is important for evaluating the quality of a device-under-test (DUT), because it represents the characteristic impedance of the transmission line under which the scattered waves from the DUT are observed. Thus, the value of $Z_{\mathrm{r}\mathrm{e}\mathrm{f}}$ should be clearly known and defined, or the calibration results will be irrelevant. In the current USB and PCIE Specifications, however, the $Z_{\mathrm{r}\mathrm{e}\mathrm{f}}$ is sometimes not clearly defined which can lead to ambiguities in the calibration process. In this paper, the influence of a small change in $Z_{\mathrm{r}\mathrm{e}\mathrm{f}}$ on the calibrated S parameters is analyzed, and experiment is carried out to show the significant impact on return loss. The results reveal the importance of specifying the value of $Z_{\mathrm{r}\mathrm{e}\mathrm{f}}$ precisely, and the characteristic impedance of the calibration kits should be regulated strictly.