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14 The PCB Design Magazine • May 2015 and dielectric constant. Then if you also need to include differential imped- ance, the trace clearance also comes into play. Plus, one needs to also consider what the preferred fab shop has in stock. So determining the correct variable for your ap- plication is not as simple as clicking an impedance "goal- seeking" button. But rather, one should weigh up all the pros and cons of changing each variable and make an informed decision. That is exactly what the impedance plots allow you to determine. Also, as the dielectric constant and loss of all ma- terials varies with frequency, the impedance needs to be simulated at the frequency of the highest bandwidth tak- ing into account the 5 th har- monic. Traditional, dielectric constant and loss has been measured at 100MHz but these days a 1GHz (or higher) frequency is more appropriate to be used to determine the impedance. In Figure 3, a data rate of, say, 400MT/s or a fundamental frequency of 200MHz can be used to determine the maximum bandwidth. From CONTROLLED IMPEDANCE DESIGN continues Figure 2: The ICD stackup Planner's impedance plots. Figure 3: Calculation of effective dielectric constant and dielectric loss. beyond design