Behavior of Short Pulses on Tightly Coupled Microstrip Lines and Reduction of Crosstalk by Using Overlying Dielectric

Forward crosstalk can be a problem in the design of printed circuit boards. We explore two aspects of forward crosstalk in tightly coupled microstrip lines. First, we demonstrate, as far as we are aware, the first direct experimental verification of different velocities of propagation in tightly coupled microstrip lines. (These are the even and odd modes for symmetric lines.) Second, we verify by modeling and by experiments the reduction in forward crosstalk when microstrip lines are covered by the proper overlying thickness of dielectric and the simultaneous reduction in differences in modal velocities. In 1981, Anderson introduced a clever means to reduce forward crosstalk in microstrip lines to zero by burying the lines under a relatively thin dielectric layer of an appropriate thickness. At this critical overlayer thickness, the capacitive and inductive crosstalk components are equal even for inhomogeneous media. Anderson's work is not well known to most packaging engineers because it appeared only in the patent literature and in the
review of scientific instruments
, an excellent journal, but not one usually read by the packaging community. We extend Anderson's results by demonstrating that at the critical thickness of the overlying dielectric the velocities of the two modes become equal. We verify that Anderson's argument extends to the case where the lines are so tightly coupled that the modal velocities differ by more than 10%. We also generalize his results to the case where the lines are asymmetric. Experimental results on microstrip lines approximately 61 cm in length confirm the modeling predictions.