The PCB Design Magazine

PCBD-Feb2014

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38 The PCB Design Magazine • February 2014 minators close to the source to dampen the re- flections. Points to Remember • A series terminator is best for high-speed transmission lines. • Bifurcated transmission lines—traces that are split into two or more T-sections—are some- times used to distribute signals to multiple loads. • At the branch, the two 50 ohm traces in parallel equate to a 25 ohm equivalent trace— and a mismatch in impedance. • Trace width is not directly proportional to impedance. • Bifurcated transmission lines with matched impedances are a better choice. • The end terminators ensure that the trans- mitted pulse progresses once down the line and then stops, preventing reflection. • Matching branched lines are not often used due to the difficulty of fabricating traces of widely varying impedance on the same sub- strate. • If one of the bifurcated traces is located on a different layer to the other, the return current will be referenced to different planes potentially creating a large loop area and EMI. • Fly-by topology employs a series termina- tor close to the driver and a parallel 100 ohm VTT pull-up at the end of the loads. • Star routing is ideal for distributing clocks to multiple loads and is also used for power dis- tribution on signal layers. • Reflections occur if the trace lengths of star routing are unmatched. • The choice is this: Match the lengths or use individual series terminators, close to the source, to dampen the reflections of a star route. PCBDESIGN References 1. Barry Olney—Advanced Design for SMT, Beyond Design: Impedance Matching: Termina- tions, Beyond Design: Differential Pair Routing, PCB Design Techniques for DDR, DDR2 & DDR3, Part 1, PCB Design Techniques for DDR, DDR2 & DDR3, Part 2 2. Howard Johnson—High-Speed Digital Design 3. Mark Montrose—EMC and the Printed Circuit Board 4. The ICD Stackup Planner and PDN Plan- ner—www.icd.com.au Barry Olney is managing di- rector of in-Circuit Design Pty ltd (iCD), Australia. This PCB design service bureau special- izes in board-level simulation, and has developed the iCD Stackup Planner and iCD PDn Planner software. To read past columns, or to contact Olney, click here. beyond design EFFECTIVE RoUTING oF MULTIPLE LoADS continues by Real Time with... NEPCON South China energy harvesting systems offer an attractive al- ternative to battery power for portable devices. Ac- cording to a recent report from navigant Research, worldwide revenue from energy harvesting systems will grow from $283 million in 2014 to nearly $375 million in 2020. Another important application for energy har- vesting is pervasive sensor networks, where thou- sands of sensors monitor all measurable commodities, in real time. These devices diligently work to provide important information about temperature, humidity, security, machine health, structural health, and much more. As an alternative technology to power these networks, energy harvesting offers several advantag- es over conventional power supplies. The report, "energy harvesting," analyzes exist- ing and emerging energy harvesting technologies in depth, looking at nine end-use consumer and indus- trial application segments and the four most success- ful transduction methods for converting ambient en- ergy. An executive Summary of the report is available for free download here. Energy Harvesting Systems to Reach $375M by 2020

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