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PCBD-Nov2016

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32 The PCB Design Magazine • November 2016 VIAS, MODELLING, AND SIGNAL INTEGRITY a few glasses of shiraz later, my mental state had equalised. But signals aren't always so lucky. Before delving down into the electrical via, I would like you to have a think about the fol- lowing scenario on a transmission line. Bear with me: Understanding the following will help make more sense of the via scenario. The second thing I would like you to bear in mind simultaneously is that good modeling can't fix a bad design. The model can tell you where a design is weak, but if you have committed your design to product, the model can only tell you how it behaves. Some less experienced design- ers seem to fall into the trap of thinking a better model will fix something that doesn't work; it won't. It will only reassure you that the design was bad in the first place. But back to the simple transmission line. Let's take a microstrip for example; it has a sur- face line running over a return path. So, in a mind experiment, let's put two major failings on this imaginary (let's say, 5 inches in length) microstrip. Remember also that when you ex- cite a transmission line with a signal, the signal propagates down the line at roughly half the speed of light (a bit more in a microstrip). In our imaginary microstrip, I'd like you to imag- ine you have cut out a slot in the ground plane about halfway along the trace and at right an- gles to it, for whatever reason. What's going to happen to your transmission line? As the signal propagates along the trace, ev- erything is normal—an equal and opposite cur- rent returns to the source in the ground plane immediately under the outbound signal. In front of the signal the line is quiet. Nothing is outbound (it hasn't gotten there yet) and no re- turn; there's nothing to return. When the signal traverses the slot, well, everything gets messed up as there is no immediate return path. You have just created a slot antenna and the cur- rent attempts to find the path of least imped- ance back to the source (which might be in the air). This is poor design, but I still see it happen from time to time. To imagine just how ridicu- lous this is, let's swap the scenario, making the ground continuous but at halfway along the surface trace, we have an open circuit for half an inch. "That will never work," I can hear you say. "It's an open circuit!" But in this scenario, some less experienced designers were happy to put an RF open circuit in the return by cutting an aperture below the outbound trace. Here is my visualisation of a side view of the transmission line. With an overdose of interpre- tation, I have imagined that the signal energy is stored in the substrate by stretching it like a piece of elastic. The signal propagates along the copper by storing energy in this imaginary medium by stretching the molecules so it expands… clear- ly some fantasy is involved here, but it should help you visualise the signal wavefront travelling through the transmission line, and what hap- pens when the return path is disrupted. Figure 1: An imagined view of pulse "stretching" the dielectric. The numeral 1 denotes stored energy. Figure 2: The energy propagates along the line. The numeral 2 denotes the direction of the current.

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