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Design007-Feb2019

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16 DESIGN007 MAGAZINE I FEBRUARY 2019 • The propagation speed of microstrip is always faster than stripline—typically, by 13-17% • Dissipation factor is a parameter of a dielectric material that quantifies its inher- ent dissipation (loss) of electromagnetic energy • Dielectric constant and dissipation factor contribute to the frequency dependent loss and to degrade the bandwidth and speed of the signal • High-frequency dielectric materials gen- erally have low Er and Df, enabling the signals to propagate faster, have less loss, and therefore, higher bandwidth DESIGN007 References 1. Firger, J. "Tiny biosensor patches worn on skin show big promise," CBS News, April 4, 2014. Further Reading • Browne, J. "Picking Materials for Multilayer PCBs," Microwaves & RF, February 17, 2011. • Morrison, R. Fast Circuit Boards: Energy Management, Wiley, January 2018. • Olney, B. "It's a Material World," Design007 Magazine, August 2018. • Olney, B. "Signal Flight Time Variance in Multilayer PCBs," The PCB Design Magazine, December 2017. • Olney, B. "Beyond Design: Microstrip Coplanar Wave- guides," The PCB Design Magazine, March 2017. • Olney, B. "Faster Than a Speeding Bullet," The PCB Design Magazine, February 2016. • Olney, B. "Controlled Impedance Design," The PCB Design Magazine, May 2015. • Olney, B. "Material Selection for SERDES Design," The PCB Design Magazine, September 2013. 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 read past columns or contact Olney, click here. When we design a transmission line, as part of a multilayer stackup, we are not just de- fining the copper traces but also specifying the dielectric to transfer the electromagnetic energy. The traces and vias guide the energy through the substrate. A field solver uses the combined effects of trace width, clearance, and thickness plus the dielectric constant and material thickness to determine the imped - ance of the trace. However, the speed of prop- agation is independent of trace geometry and is totally determined by the dielectric material. All dielectric materials that compose a sub- strate have different properties, so one needs to select the optimal materials for the required purpose carefully. Key Points: • The impedance of transmission lines is a function of substrate dielectric constant • The CTE is a yardstick for expected PTH barrel reliability • Adding a coupled return path close to the signal conductor in the presence of a dielectric reduces the impedance • Typically, for a digital design, a characteristic impedance of 40–60 ohms and differential impedance of 80–120 ohms are used • A good transmission line is one that has constant impedance along the entire length of the line, so that there are no mismatches resulting in reflections • Coplanar impedance is determined by the ratio of trace width to clearance, so size reduction is possible without limit—the only penalty being higher losses • A transmission line does not carry the signal itself but guides electromagnetic energy from one point to another through the substrate • With their relative timing requirements, the signals essentially ride the EM carrier wave • The speed of a computer does not depend intrinsically on the speed of electrons but on the speed of energy transfer between electronic components

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