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.Comment: Code on github:
https://github.com/ZiadHatab/verification-multiline-trl-calibratio