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Design007-Jan2018

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94 DESIGN007 MAGAZINE I JANUARY 2018 tion network reduce the impact of simultane- ous switching noise and electromagnetic radia- tion in high-speed digital PCB designs. Key Points • Supply bounce is fundamentally related to the total inductance of the current path or shared return paths. It is the primary cause of simultaneous switching noise and electromagnetic radiation. • When supply bounce occurs, the charge that is impressed across the power delivery path results in common-mode voltage. It is this common-mode voltage that creates electromagnetic emissions. • The magnitude of the radiated peaks can be limited by providing a very low AC impedance path between power and ground. • Supply bounce interferes with the recep- tion of the signal at the load, depending on the noise margin, and can cause double clocking. • Supply bounce gets worse as the result of increased lead inductance, capacitive load and simultaneously switching outputs. It also deteriorates with reduced resistive load. • At high-speeds, the lead inductance of an IC package is critical. Larger packages tend to have more lead inductance. • A number of approaches can be imple- mented during layout and routing of the PCB to minimize the voltage drop, hence supply bounce, in the power delivery path. References 1. Barry Olney's Beyond Design column, The Dumping Ground. 2. Understanding and Minimizing Ground Bounce, Fairchild Semiconductor. 3. EMC and the Printed Circuit Board, Mark Montrose. 4. Signal and Power Integrity – Simplified, Eric Bogatin. 5. High-Speed Digital Design, Howard Johnson. Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board-level simulation. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN and CPW Planner. The software can be downloaded from www.icd.com.au. To contact Olney, or read past columns, click here. A method developed by the Rice lab of chemist Matteo Pasquali allows researchers to make short lengths of strong, conductive fibers from small samples of bulk nanotubes in about an hour. The work comple - ments Pasquali's pioneering 2013 method to spin full spools of thread-like nanotube fibers for aerospace, automotive, medical and smart-clothing applications. The fibers look like cotton thread but perform like metal wires and carbon fibers. It can take grams of material and weeks of effort to optimize the pro - cess of spinning continuous fibers, but the new method cuts that down to size, even if it does require a bit of hands-on processing. Pasquali and lead author and graduate student Robby Headrick reported in Advanced Materials that aligning and twisting the hair-like fibers is fairly simple. First, Headrick makes films. After dissolving a small amount of nanotubes in acid, he places the solution between two glass slides. Moving them quickly past each other applies shear force that prompts the bil - lions of nanotubes within the solution to line up. Once the r esulting films are deposited onto the glass, he peels off sections and rolls them up into fibers. Pasquali said the process repro - duces the high nanotube alignment and high packing density typical o f fibers produced via spinning, but at a size suffi - cient for strength and conductivity tests. Nanotube Fibers in a Jiffy

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