Printed Circuit Design & Fab - December 2007 - (Page 28)

RF DESIGN that if this return path is not where you want it to be, you can (most likely will!) get into trouble. This means that, to be successful, we must change from “indirect” creation of return paths to actively considering return paths in the design work. For every routed signal, we have to ensure that we have a proper return path that is located where we want it and that we don’t obstruct already created paths, because signals sharing a part of the return path have common impedance coupling (energy coupled from one signal to the other). The larger the area spanned by signal and return path, the larger the radiation and susceptibility. (EMI is proportional to the loop area.) find its return path as the path of least capacitance. This makes things extremely complicated as the path of “least capacitance” is not as predicable as for inductance. It may very well include any conductive surface in proximity, such as shield boxes or a metal cabinet. At the same time, the return path is really critical to circuit function. We need to establish where the path is and that it is not shared for two sensitive signals to avoid common impedance coupling between them. The key to controlling indirect coupling is proper grounding. Obviously, we don’t want RF and digital (or analog) to share return paths. This is why we work with separate grounds. Connecting all ground in a single point helps ensure that the return paths are not shared between the systems. However, within the same ground system, we need to manage the return paths -- keeping in mind the rule that it will be the path of least impedance for the frequency with which we are working. Where is the Return Path? At DC, the return path is a straight line (path of least resistance), but as frequency increases, we cannot simplify and just say “the path of least resistance” anymore. It is in fact the path of least impedance. At the low to medium-high frequency of the digital high-speed spectrum, inductance is the dominating part of impedance. Therefore, we can somewhat simply say that the return path is the path of least inductance. In today’s circuit boards, this path is usually in the ground plane right under the signal conductor. This deviates only if there are obstacles in the form of a slotted or split ground plane or where vias perforate the ground plane enough to cut the return path. At RF frequencies (this includes ultra-high-speed digital), capacitance becomes dominant. Thus, the signal will typically Power Feed Leaks Return path and separate grounding helps, but in an RF system, we also need to ensure that RF energy must not leak onto the power supply itself and radiate or cause other problems there. When I started doing RF design, we used closed metal cabinets for each module, with each cabinet carefully grounded. We used so-called feed-through capacitors for every signal or power entering or leaving the shielded compartment. This was a conductor with a coaxial capacitance to ground intended to “Our webinar was extremely successful— ADVERTISER INDEX To learn more about the advertisers in this issue, go to pcdandf.com and select “Advertiser Index” in the home page menu. This will provide you with a direct link to the Web site of each advertiser in this index. professionally managed, executed to plan and ﬂawless. The experience exceeded my expectations.” Michael Kirschner, President, Design Chain Associates LLC ADVERTISER Accutrace American Standard Circuits Bare Board Group Beta Layout CADParts & Consulting CAM Expert CEPD Circuitaid+ Circuitronix Compufab Current DesignCon 2008 PAGE # 46 45 47 33 29 46 46 46 45 47 35 39 1 46 3 45 44 13 44 7 5 C4 ADVERTISER PAGE # 45 C2 11 44 Midwest Accurate Grinding National Instruments/ Electronics Workbench Group NEC Information Systems Online Electronics Operations Technology (OPTEK) 25 OverflyPacific Corp. Papros PCB East 2008 PCB Shows PCB FAB Express Precision Technologies Rogers Samtec Sierra Proto Express Simberian Superior Processing T-Tech UPMG Webinars 46 46 30 20 47 44 8 21, 27 C3 29 45 23 28 The possibilities are unlimited with Webinars by UP Media Group For more information on how we can help you meet and exceed your marketing objectives, contact: Frances Stewart fstewart@upmediagroup.com 678-817-1286 DownStream Technologies eFabPCB EMA Design Automation EzPCB Fine Circuits HMS Hollmuller Maschinenbau GmbH Imagineering Intercept Technology Mentor Graphics – PADS Mentor Graphics – Expedition Valor Computerized Systems Insert opposite p. 17 Virtual PCB Wise Software 15 37 www.pcbshows.com/webinars The advertising index is published as an additional service. The publisher does not assume any liability for errors or omissions. 28 PRINTED CIRCUIT DESIGN & FAB DECEMBER 2007 http://pcdandf.com http://pcbshows.com/webinars

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Printed Circuit Design & Fab - December 2007 Contents Our Line Market Watch Around the World Happenings ROI 2007 PCB Designer Salary Survey Interconnect Strategies Positive Plating RF Design Ad Index Noise Reduction Supply Chain DfM Off the Shelf Marketplace The Signal Doctor

Printed Circuit Design & Fab - December 2007

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