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60 DESIGN007 MAGAZINE I FEBRUARY 2021 e first two edge coupled configurations are well known—and for good reason—they are also commonly used for differential pairs. Close coupling is good for maintaining imped- ance, rejecting noise and avoiding slew, but not good for crosstalk. e other two con- figurations are broadside coupling, which has many disadvantages: the main one being layer- to-layer registration during fabrication. IPC Class 3 specifies ±2 mil registration between any two adjacent layers but it may be more like ±4 mil for a combination of layers nudging the impedance way out. Also, broadside cou- pling requires a very thick dielectric material between adjacent signal layers which will gen- erally render the substrate too thick. On the plus side, it can be a good solution for navigat- ing a connector pin array, maintaining imped- ance with no reference place. Broadside-edge coupling requires less dielectric thickness but is very difficult to predict the impedance due to alignment issues. Impedance aside, any cou- pling of unrelated signals should be avoided. ere are two types of crosstalk: forward and reverse. ese are also termed far-end (FEXT) and near-end (NEXT) crosstalk, which refers to where the crosstalk is measured—at the load or the receiver, respectively. Interestingly, there is a unique property of the stripline configuration in that the ratio of mutual capacitance (Cm) equals that of the mutual inductance (Lm) which cancels out the forward crosstalk (Kf) component (equation 1). Whereas reverse crosstalk (Kb in equation 2) is the addition of the two, consequently it is always present to some degree. equation 1 Forward crosstalk (FEXT) equation 2 Reverse crosstalk (NEXT) Figure 2 shows the near- and far-end cross- talk for a microstrip configuration where the victim traces are adjacent to the aggressor Figure 2: Near- and far-end crosstalk for microstrip with 4/4 mil trace width/clearance.