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Design007-May2019

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22 DESIGN007 MAGAZINE I MAY 2019 High-speed PCB Design Constraints Digital design has entered a new realm. Mod- ern high-speed design (HSD) not only requires the designer to continuously break new ground on a technical level but also requires the de- signer to account for significantly more vari- ables associated with higher frequencies, fast- er transition times, and higher bandwidths. Ig- noring signal and power integrity and electro- magnetic compatibility invites schedule delays and increases development costs and the pos- sibility of never succeeding to build a function- al product, which is a career-limiting strategy. The key methodology is to understand the underlying high-speed design issues and then translate these into corresponding design con- straints that will be adhered to during the en- tire design process. It is best to develop these high-speed design constraints based on pre- layout simulation (Figure 1). We had a few critical nets to manage in the past, but now, it seems that a significant num- ber of interconnects are critical. Also, each de- sign requires a specific set of constraints based on the technologies used. Sure, we can port basic design rules for trace width, clearance, etc., from a previous design to the next, but individual constraints still need to be estab- lished. Constraint reuse is also limited by net and group naming conventions. If you are con- sistent, then porting is much easier. Beyond Design Feature Column by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Develop high-speed design constraints based on pre-layout simulation (xDX Designer to HyperLynx).

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