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62 DESIGN007 MAGAZINE I DECEMBER 2019 is mainly used to determine the characteristics of a given electrical line by observing reflect- ed waveforms, they can also be used to locate discontinuities in a connector. In the case be- ing shown regarding a large braid of steel cable encased in concrete, some of the back-filling of the concrete did not fully encapsulate the wire bundle, leading to areas of moisture en- trapment and, therefore, oxidation of the wire strand, causing a failure of the bridge. So, why the Sunday morning revelation? I began to think about how small changes in design characteristics at the PCB fabrication level can have larger consequences for the fi- nal product. Some of these include changes in trace geometry, dielectric, material type, cop- per weights, etc. Let's go through a basic list of them and discuss each one. 1. Change From "Free Space" to a Coplanar Coupled Trace Figure 1 shows a free space calculation. Fig- ure 2 shows an added copper pour, creating a coplanar model. As you can see, with the added coplanar coupling, characteristic impedance went from 49.86 ohms to 43 ohms for a 50-ohm trace due to poured copper close to the affected traces. Let's take that one step further with the next example. 2. Change in the Mask Color From the Original Calculations Using that last example, let's make things even more complicated. For instance, let's as- sume that the customer has changed the free Figure 1: Free space calculation for a .00675" 50-ohm trace. Figure 2: The same trace with added copper pour.

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