  This paper presents a new technique for evaluating the consistency of the
reference impedance in multiline thru-reflect-line (TRL) calibration. During
the calibration process, it is assumed that all transmission line standards
have the same characteristic impedance. However, these assumptions are prone to
errors due to imperfections, which can affect the validity of the reference
impedance after calibration. Our proposed method involves using multiple
stepped impedance lines of different lengths to extract the broadband
reflection coefficient of the impedance transition. This reflection coefficient
can be used to validate the reference impedance experimentally without
requiring fully defined standards. We demonstrate this method using multiline
TRL based on microstrip structures on a printed circuit board (PCB) with an
on-wafer probing setup.
