An Impedance Transition Method to Verify the Reference Impedance of Multiline TRL Calibration

In this paper, we present a new technique for assessing the validity of the reference impedance in multiline thru-reflect-line (mTRL) calibration. When performing an mTRL calibration, it is assumed that all transmission line standards exhibit the same characteristic impedance. As a result, the reference impedance after calibration is set to the characteristic impedance of the transmission line standards used in the calibration. However, because of imperfections, these assumptions are prone to errors. The purpose of this paper is to assess the validity of the reference impedance after an mTRL calibration. The method we propose uses the reflection coefficient of an impedance transition segment as a verification metric. The verification is achieved by performing two mTRL calibrations. The first mTRL calibration is the one we desire to validate, while the second mTRL calibration is based on step impedance lines that create the impedance transition. We conclude that the mTRL calibration is valid if the resulting reflection coefficient falls within the expected 95% confidence interval. We demonstrate our proposed method with printed circuit board (PCB) measurements of microstrip lines up to 150 GHz. The advantage of our approach is that the reflection coefficient of an impedance transition is almost constant with respect to frequency for many types of transmission line, which makes this validation metric easy to interpret when errors are present.