Printed Circuit Design & Fab - December 2007 - (Page 29)

short circuit any RF energy trying to escape. We also used RF chokes made of a quarter-wavelength of wire wound as a coil. Connected in a series with power or LF lines, they acted as a high-impedance obstacle to further limit RF leaks. Today, this is too cumbersome of an approach, and integrated board compartments are more practical replacements. We still need to decouple interconnects very carefully to prevent RF leaks via power and low-frequency lines in the design. FIGURE 3. Yuasa, et. al. derived a formula for determining the center frequency of the pass band for the band pass filter created by the via a regular peppering pattern matrix. Ground Fences In RF system designs, we frequently see each module in the design encapsulated by a narrow ground guard stitched with via holes: either a single row of vias or a dual staggered row. When designers are asked why they use a certain via spacing or pattern, the answer often includes wind direction and shoe size. Luckily, some work has been published on this topic; so there is no reason to resort to black magic. We can use via fencing for both power lines and signal lines. As we all know, currents in power planes are overlaid with transients. This can make the power bus radiate the same way as a microstrip patch antenna1. Xin Wu et al. suggest that setting the via spacing in the fence to less than 1/10th wave length can give up to 25 dB suppression compared to no fencing1. You would think that the closer a via fence is placed to an RF line the better. However, when placed too close, the radiation losses increase. This will actually increase the coupling2. As a rule of thumb, the via fence must be set away from the RF trace by at least four times the distance between the signal layer and the ground layer of a multilayer PCB2. FIGURE 1 shows the field distribution for a stripline with widely spaced via fencing. The vias are 1X height over ground away from the strip line, and the via spacing is 5.2X height over ground. The radiation losses are clearly visible. FIGURE 2 shows the same strip with a much narrower via spacing of 1.3X height over ground. The radiation losses are significantly lower here. Dr. Ponchak (recommended reading on this topic!) suggests that the smaller the via spacing, the better2. A solid wall would be ideal if it could be manufactured. In RF systems, we also need to manage the ground and power planes. We have already covered that the currents on these planes have a transient component and that transient currents through a conductor mean radiation, which we obviously don’t want. This leads us to via peppering of Build 3D electromagnetic models for interconnects and via-holes A leader in electronic and product design services. Specializing in Rapid Prototype Design Fab, and Assembly. We have a proven track record for turning concepts into products quickly and accurately. Design FAB Assembly Product Design Services Prototype to Production: Engineering Schematic Capture PCB Layout Tool Sets Allegro Concept Capture-CIS Expedition DxDesigner PADs Logic PADs Layout Cadstar Sch & PCB Introducing Simbeor™ 2007 • 3D full-wave analysis • Metal surface roughness • Broadband dielectric models • Dispersion and edge effects • Skin and proximity effects • Fits into any design flow • Accurate via-hole models • 3D transmission line models • RLGC and S-parameters • Via-holes design wizard Now Serving Customers Coast to Coast NJ Design Center, Morganville, NJ (732) 972-0247 TN Design Center, Crossville, TN (931) 788-6312 CA Design Center, Sunnyvale, CA Download a free trial at www.simberian.com 1 (877) 247-CADP (2237) © 2007 Simberian Inc. Visit us on the web www.cadpartsusa.com DECEMBER 2007 PRINTED CIRCUIT DESIGN & FAB 29 http://www.cadpartsusa.com http://www.simberian.com http://www.simberian.com http://www.cadpartsusa.com

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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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