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Dr. Howard Johnson, the world’s foremost authority on signal integrity, has recently released his High-Speed Digital Design (HSDD) Collection. This includes professionally recorded seminars that he presented, for more than 20 years, at Oxford University and worldwide and is arguably the most practical and enlightening course on high-speed—black magic—ever delivered. Howard’s unique explicatory presentation style creates an unforgettable picture of signal propagation by practical example. If you want to gain some of his enthusiasm and master the art of high-speed design, then the collection is a must-have.
I recently had the opportunity to review all three of the seminars in this collection, a total of 36 hours of viewing time. When presented with a selection of three seminars, to watch, I guess it is only natural to want to start with the more advanced topic. But I am glad that I forced myself to start at the beginning to refresh the basics before moving on to the more complex issues. It is amazing how much I either did not know or had simply forgotten over the years. Or maybe I’ve just killed too many brain cells along the way!
I know that during my own courses, there is always one guy at the back who falls asleep. And strangely enough, he is always the one to give bad feedback, probably because he didn’t learn anything. But I guarantee that you will be on the edge of your seat throughout Howard’s entire seminar series. His dynamic teaching style ensures you feel like you are not just a part of the audience—you are actually participating in the demonstrations. The picture that Howard paints leaves a lasting impression on how electromagnetic fields propagate and how they induce voltages and current (crosstalk) into nearby signals. The following is a section-by-section discussion of the course contents.
1. High-Speed Digital Design
Engineers and PCB designers need to understand electromagnetic theory, appreciate how coupling occurs and why energy moves to unintended, sensitive parts of the circuits. A logic schematic diagram masks details crucial to the operation of unintentional signal pathways vital to your understanding of signal performance, crosstalk and EMI. To realize these factors, one must uncover the hidden schematic, operating behind the logic diagram, to reveal the parasitic elements that affect the circuit. These parasitics are invisible to the uninitiated, but become very clear once skillfully explained in detail. You will gain new insight into what really happens in the circuitry.
Also, understanding the frequency band that really matters for digital design is very important. Traditionally, we used 0.35/Tr (where Tr is the rise time in ps) for the upper bandwidth. However, Howard recommends using an upper knee frequency of 0.5/Tr, which forms a crude, but useful, translation between time and frequency domains. So for instance, if the rise time is 500ps, which is typical these days, then the upper bandwidth is actually 1GHz regardless of the clock frequency. Furthermore, the constant improvement in the IC process reduces die size which speeds-up the rising edge. This in turn pushes the knee frequency up, causing signal overshooting and ringing.
To read this entire article, which appeared in the June 2016 issue of The PCB Design Magazine, click here.