Effect of Dielectric Permittivity Non-Uniformity on Microwave Far-End Crosstalk in Coupled PCB Microstrip Transmission Lines

Far-end crosstalk between coupled microstrip lines with varying spacing is analyzed. The difference between the capacitive and inductive coupling coefficients is used to quantify this crosstalk, as it is directly proportional to this difference. It is demonstrated that, due to the presence of the solder mask, the far-end crosstalk does not always increase as the spacing between lines decreases, but it follows non-monotonic behavior. Considering this finding, data calculated from the coupling coefficients versus spacing is then used to determine the optimal spacing between a single-ended channel and a differential pair channel, in which the far-end crosstalk is minimized. Furthermore, experimental results show that the contrast between the PCB dielectric laminate and the solder mask causes the crosstalk to significantly differ in magnitude for microstrips with identical cross-sections, highlighting the importance of selecting the right combination of solder mask and dielectric laminate as a key design parameter for applications with stringent crosstalk noise specifications.