Wideband extraction of Dk and Df of printed circuit boards using two transmission lines with sufficient length difference

Two striplines that have identical cross-section and sufficient length difference were used in this investigation as a test vehicle to extract permittivity of printed circuit board. The extraction procedure for Dk and Df follows the algorithm of conventional TRL calibration to firstly extract the frequency-dependent propagation constants of the test vehicle; then the values of Dk and Df at each frequency point are extracted from them by the aids of empirical models of stripline structure. The issue of limited frequency span resulted from numerical ill conditions that generally occurred for TRL-calibration procedure was significantly reduced by introducing sufficient line loss. Increasing the length difference of the test vehicle would introduce more line loss and stabilize the recovering procedure; therefore the extraction bandwidth was immensely increased. Since only two signal-ended striplines are used, this procedure could also save the estate of PCB occupied by the test vehicle, time and cost required for the test measurement. This procedure is also suitable for characterizing temperature-dependent variations of Dk and Df since vector network analyzer calibration is not needed, and un-calibrated S-parameters of the two test lines can be used to correctly extract the permittivity of PCB if proper successive average was applied.