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Design007-Dec2020

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DECEMBER 2020 I DESIGN007 MAGAZINE 57 • Decoupling and bypass capacitors supply instantaneous current at different frequen- cies to the drivers until the power supply can respond • Decoupling capacitors also lower the impedance at different frequencies to help meet the AC impedance target • PDN design is a trial-and-error process and needs to be done with the assistance of a PDN analysis tool • If you have a simulation tool that is built into the PCB design software, then critical signals can be readily simulated by extracting the topology into the simulation environment • Crosstalk is typically picked up on long parallel trace segments on the same layer but may also be broadside coupled from the adjacent layer • One could compare the matched lengths of each signal, but the delay will vary depending on the meander pattern and stackup layer. Thus, it is important to match delays, not lengths DESIGN007 Further Reading • B. Olney, "Beyond Design: Pre-Layout Simulation," The PCB Magazine, July 2012. • B. Olney, "Beyond Design: Intro to Board-Level Simu- lation and the PCB Design Process," The PCB Magazine, March 2012. • B. Olney, "Beyond Design: Board-Level Simulation and the Design Process: Plan B—Post-Layout Simulation," The PCB Magazine, February 2012. • B. Olney, "Board-Level Simulation," Electronics News, Australia, February 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 at icd.com.au. To read past columns or contact Olney, click here. If, for instance, on the third iteration, your product still does not meet specifications, and management is pushing to have the project completed, then you may need to look for new employment opportunities. However, if simu- lation is done during the design process, then these re-spins can be avoided, keeping your project on budget and on schedule. Key Points • The product life cycle is very short; therefore, time to market is of the essence • Multilayer boards can be designed to work right the first time with little additional effort, providing you follow a tried and proven process • Re-spins will continue to happen until designers make regular use of circuit simulation software • Simulation is often engaged toward the end of the design cycle, which is too late • Ideally, the simulation should be done during the design process as part of standard practice • A pre-layout simulation identifies issues in the conceptual stage • The post-layout simulation catches the issues during the design process, eliminating the potentially disastrous final stage changes • Keep your eye on the ball during the entire design process, catching any small issue before it becomes a major problem • Before starting a PCB design, you need to plan the PCB stackup and ensure that the selected substrate materials are available from your chosen fabricator • If you use the same materials that the fab shop stocks to build your stackup, then the impedance will be more accurate • The design of the PDN is also a very important part of the conceptual design process, ensuring that you have a stable power delivery system

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