Software Bytes: Having Fun With Impedance


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About a year ago, I was assigned a new project: become an expert in impedance, more or less. I had no idea how much this research would bring out the nerd in me. I’m still not an impedance guru, but I’ve learned a lot about how impedance requirements affect PCBs.

Even if you don’t typically design controlled-impedance circuit boards, you probably will eventually. Fortunately, designers have access to impedance software that takes care of the heavy lifting.

Let’s start at the beginning. According to Wikipedia, “Electrical impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.” Makes perfect sense, doesn’t it?

Obviously, the definition of impedance doesn’t really tell us what it means to designers and engineers. As high-speed designs continue to push into ever faster envelopes, the tools used to analyze electromagnetic interference (EMI), signal integrity (SI), and impedance are becoming more and more crucial to executing a successful, sustainable design.

As an electrical engineer co-worker said, “It’s not whether the design works. It’s whether it can work better.” A great example of this is how the airlines are now starting to allow cell phone usage in the sky. With aeronautical equipment and mobile devices becoming more streamlined, with far less noise emitted in either direction, there are fewer chances that the frequency of a cell phone will collide with the frequency of the airplane’s instruments. Providing this convenience to passengers is a milestone not only for the airlines, but also for all of the software programs that support the careful, iterative improvements made upon older design concepts. (Just don’t talk non-stop on your cell phone if you’re sitting next to me.)

So, you’ve put the finishing touches on some of your beautiful hand routes, only to have the engineer come ask you to switch out your resistors for a different value, and space out your wires by .005” instead of .004”. Don’t take it personally; your engineer is just trying to preclude any impedance issues.

To read this entire article, which appeared in the July 2016 issue of The PCB Design Magazine, click here.

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