Estimation of strip-line losses in printed circuit boards

As computer processing speeds increase into the GHz frequencies, the dielectric and conductor losses in a strip-line, which are negligible at data rates below 1 Gbps, become increasingly important as they lead to a decline in signal integrity. The dielectric losses of printed circuit boards (PCBs) are computed from measured loss tangent values, which are very small and whose accuracy is not clearly defined. Therefore, a more accurate generalized model of a slightly lossy dielectric is needed to better estimate losses. As measured values of the real permittivity component are more accurate, they can be used to estimate the lossy component using a modified form of the Kramers-Kronig relations. Using Collin´s relationships for a strip-line, the conductor losses are estimated. The sum of these theoretically calculated losses for TEM propagation agreed well with measured losses for short strip-lines in PCBs, from 100 MHz to 6 GHz. It is also seen that there is a comparatively small deviation of 1.97 dB for higher frequencies (6-14 GHz).