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MARCH 2019 I DESIGN007 MAGAZINE 63 (b) However, for the stripline configuration, one needs to also take into account the ra- tio of h1 (height of the plane above the trace) compared to h2 (height of the plane below the trace). Then, h for the plane above becomes: And for the plane below, h is: (c) You can easily extrapolate these equa- tions to accommodate dual asymmetric strip- line by adding the height of the appropriate dielectrics to each plane; (h1+h2) becomes (h1+h2+h3) in the previous equations. This will result in a similar current distribution to that shown in Figure 3. Also, to be more accurate, in the stripline configurations, the trace thickness (t) sinks into the prepreg material, bringing the trace closer to the plane and reducing the trace impedance. So, accounting for this resin flow adds a little more complexity to the equation. However, given that you know which dielectric material is core and which is prepreg, the height of the prepreg can be re- duced by (t). The fundamental distribution of the current equation is also the basis for simple cross- talk estimates. Crosstalk changes very rap- idly with distance and plummets roughly quadratically with increased separation (d) or decreased dielectric height (h). For mi- crostrip: Crosstalk is expressed as a ratio of noise volt- age to the driving signal amplitude. The con- stant (k) depends on the circuit rise time and the length of the interfering trace segments. This is always less than one. Surprisingly, these equations are based on Newton's 300-year-old inverse-square law: the force acting between two objects is in- versely proportional to the square of the sep- aration distance. It always amazes me how math, physics, electromagnetics, and oth- er disciplines all align with the exception of subatomic quantum theory, which does not seem to comply with any established law of nature. Conclusion When modeling a trace above a solid plane, you will find that the current density is great- er on the reference plane side of the trace than on the other. The same principle applies for two traces placed in close proximity in paral- lel segments—the current tends to concentrate on the two facing edge surfaces. The proximity effect is a simple manifestation of the general rule that high-speed current tends to concen- trate near its return path. Figure 5: (a) Microstrip, (b) stripline, and (c) dual stripline configurations.