Effect of via stubs on the TRL calibration technique for measurement of embedded multilayer structures

In this paper the effect of access via stubs on the Thru-Reflect-Line (TRL) calibration for deembedding of multilayer printed circuit board structures is analyzed. For this purpose a dedicated calibration board with single ended TRL standards containing broadband signal launches for measurement of embedded (internal) printed circuit board (PCB) structures such as striplines and vias (plated through holes) was designed. First results from the extracted signal launch S-parameters in the bandwidth from 40 MHz to 40 GHz using two-tier calibration techniques are shown and corroborated using full-wave 3-D FEM solver models. It was found that the access via stubs represent highly resonant structures that act as short circuits at their quarter wavelength frequency (and odd multiples thereof). As a consequence the TRL algorithm fails at these frequencies and the calibration cannot be accomplished. Several mitigation strategies like using a recessed probing or backdrilling are discussed and their extension of calibration bandwidth is quantified.