Improvements in Time-Domain TRL Accuracy for Transmission Measurements

This paper presents the analysis of the two types of possible errors associated with time-domain TRL calibration. The first one is time-base distortion, the second one is diode-state error or switching error. The paper describes the time-base calibration procedure and the method to remove the time-base distortion from the measurement. It has been shown that the time-base distortion is responsible for an offset in the insertion loss measurement relative to the VNA reference. The switching error can be removed by a two-tier calibration of the time-domain vector network analyzer, however, the application of the two-tier calibration does not lead to the accuracy improvement and indicates that the remaining errors are caused by other factors. One possible explanation of the observed discrepancy is the nonlinearity of the instrument. The harmonic distortion of the oscilloscope was measured and a model of the time-domain VNA with nonlinear ports was built. The nonlinear VNA model shows the distortions of the S-parameters similar to that observed in the experiment, which indicates the importance of the instrument nonlinearity to the time-domain S-parameter measurements