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OCTOBER 2018 I DESIGN007 MAGAZINE 51 • Drivers do not have the same impedance as the transmission line, so terminations are used to match the impedance • Reflections occur whenever the imped- ance of the transmission line changes along its length • Ringing can be dramatically reduced by terminating the transmission line • The higher the aggressor voltage, the more crosstalk will be induced into the victim signal • Position the fastest devices closest to the connector and be graduated in descending order of speed down to the analog sections farthest from the connector • Route fences direct the routing, which prevents signals from crossing and allows the control signals to pass • The return current path will be directly below that trace and will not wander into nearby sections • The proper layout of the PCB may mean the difference between a reliable product and a board that performs intermittently • Controlling the maximum placement of devices limits the maximum signal delay to approximately the longest Manhattan distance • Given a length constraint, a designer can manage signal integrity by controlling the PCB trace topology of the various parts of an interface Further Reading 1. Beyond Design: Stackup Planning, Part 2 by Barry Olney, The PCB Design Magazine, July 2015. 2. Beyond Design: Controlled Impedance Design by Barry Olney, The PCB Design Magazine, May 2015. 3. Beyond Design: Impedance Matching—Terminations by Barry Olney, The PCB Design Magazine, October 2013. 4. Beyond Design: Critical Placement by Barry Olney, The PCB Magazine, September 2012. 5. Beyond Design: Mixed Digital-Analog Technologies by Barry Olney, The PCB Magazine, August 2012. 6. High-Speed Digital Design: A Handbook of Black Magic, First Edition by Howard Johnson and Martin Graham, Prentice Hall, 1993. Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Aus- tralia, 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. Recipes for 3D printing, or additive manufacturing, of parts have required as much guesswork as science. Until now. Now, researchers at the National Institute of Standards and Technology (NIST) have demonstrated a novel light- based atomic force microscopy (AFM) technique—sample- coupled-resonance photorheology (SCRPR)—that measures how and where a material's properties change in real time at the smallest scales during the curing process. 3D printing, or additive manufacturing, is lauded for flexible, efficient production of complex parts but has the disadvan - tage of introducing microscopic varia- tions in a material's properties. Because software renders the parts as thin layers and then reconstructs them in 3D before printing, the physi- cal material's bulk properties no longer match those of the printed parts. Instead, the performance of fabricated parts depends on printing conditions. NIST's new method measures how materials evolve with submicrometer spatial resolution and submillisecond time resolution—thousands of times smaller-scale and faster than bulk measurement techniques. Surprising the researchers, interest in the NIST technique has extended well beyond the initial 3D printing applications. Com - panies in the coatings, optics and additive manufacturing fields have reached out, and some are pursing formal collaborations, NIST researchers say. (Source: NIST) New NIST Method Measures 3D Polymer Processing Precisely