Issue link: https://iconnect007.uberflip.com/i/691539
60 The PCB Design Magazine • June 2016 2. High-Speed Noise and Grounding The second seminar in the series focuses on interactions in the real world: electromagnetic compliancy (EMC), which encompasses radia- tion and susceptibility. Howard looks at a subset of EMC effects, specifically, coupling between circuits within one system, or between large systems in a complex product. He further delib- erates on the sort of things that cause intermit- tent, unreliable behavior of the product in the field and he advises on how to alleviate these issues before they arise. In Figure 2, Dr. Johnson measures crosstalk in his really cool giant scale BGA model, and looks at supply bounce and ground ball place- ment on the BGA package. There are also many Signal Integrity Laboratory (SI Lab) experiments embedded in the videos that really help dem- onstrate the distribution of high-frequency cur- rent on reference planes and how crosstalk de- velops where current loops overlap. And, more importantly, how to avoid crosstalk. Splitting planes and creating moat and drawbridge constructions are an effective au- dio frequency solution. But RF coupling (in Fig- ure 3) is quite different, in that it takes place through magnetic and electric fields that can easily propagate through space, going right across a reference plane cut. A sensible solution here is to increase the spacing of coupled de- vices or add solid shielding. However, you can never completely isolate system components as parasitic capacitance links them whether you like it or not. For low-impedance circuits, like high-speed computers, most of the issues are related to inductance. Whenever you deviate from a solid reference plane, returning signal current spreads far and wide forming large loop areas and creating radiation. And, mutual inductance connects nearby circuits that don't even touch each other. If you do a lot of system testing and design verification, in the lab, then this seminar is for you. The second half focuses on system level grounding, connector issues and measurement. Howard also looks at clock related noise issues, the two separate modes of operation—common MASTERING "BLACK MAGIC" WITH HOWARD JOHNSON'S SEMINARS Figure 3: Split planes do not prevent RF coupling.