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Practical Tips for Stackup Design f Keep High-Speed Signals Adjacent to Ground Planes: Placing high-speed layers next to ground planes helps control impedance and minimize EMI. f Minimize Layer Transitions: Limit the number of vias and layer transitions in high-speed signals to reduce impedance mismatches and signal reflections. f Use Symmetric Stackup Design: A symmetrical stackup helps reduce mechanical stress during manufacturing and prevents warping, contributing to a more reliable design. Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC) As frequencies and data rates increase in high-speed PCB designs, controlling electromagnetic interference (EMI) and ensuring electromagnetic compatibility (EMC) become crucial. EMI can degrade the performance of nearby circuits and, if not controlled, may cause the device to fail regulatory EMC standards. Managing EMI/EMC effectively requires careful design practices and layout strategies. Electromagnetic Interference (EMI) f Problem: EMI arises when high-speed signals radiate unwanted electromagnetic waves that can interfere with neighboring circuits or external devices. High-speed traces and components with rapid switching, such as microprocessors and memory modules, are common sources of EMI. f Solution: Set EMI constraints in the Constraint Manager and employ techniques like shielding and controlled impedance to minimize radiation. OrCAD X also has simulation tools to evaluate potential EMI issues before the final layout. f Implementation Steps: ɢ Define trace width and spacing constraints in Constraint Manager to control impedance and reduce radiated emissions. ɢ Use ground planes and shielding to contain EMI, especially around high-frequency traces and sensitive components. Electromagnetic Compatibility (EMC) f Problem: EMC ensures that the device can operate without interference from or with other electronic devices. Devices need to meet EMC regulatory standards, such as FCC or CE certification, which place limits on emissions. f Solution: Define EMC compliance constraints, ensuring that layout practices minimize emissions and comply with regulatory standards. f Capabilities in OrCAD X Presto: ɢ You may use Presto PCB Editor to place shielding and define enclosure grounding points, creating barriers that prevent interference from escaping the device. ɢ Set up trace clearance and spacing rules in Constraint Manager to ensure sensitive signals are isolated from potential EMI sources. ɢ Check for some level of EMC compliance using the Impedance or Coupling analyses tools that are integrated, allowing for early detection of areas needing shielding or rerouting. Practical Tips for Managing EMI and EMC in High-Speed Designs f Minimize Trace Loops: EMI is often generated by looped traces that act like antennas. Keep trace loops as short and compact as possible, especially for power and high-speed signals. f Use Solid Ground Planes: Continuous ground planes adjacent to high-speed signals reduce EMI by absorbing and containing electromagnetic fields. f Run Differential Pairs for High-Speed Signals: Differential pairs help balance the electromagnetic fields between traces, reducing radiated emissions. The constraint settings and parameters for differential pair routing help maintain consistent spacing and length, in the Electrical and Physical rule sets within the Constraint Manager, even for extended nets. f Shield Sensitive Components: Physically shield components that are prone to or produce EMI. Use metal shielding enclosures in the design for critical areas and ground them properly. 16 www.cadence.com OrCAD X High-Speed Digital Design Guide