OrCAD X Resources

2. Trace Width Variations: f Problem: Impedance is affected by the width of a trace, the spacing from adjacent traces, and the distance to the ground plane. Any change in width, for instance when routing around components, can cause impedance fluctuations. f Solution: Set trace width constraints to enforce a consistent width across critical high-speed paths. The Design Rule Checks (DRCs) will flag any violations, ensuring that critical traces remain uniform. 3. Proximity to Other Components or Ground Planes: f Problem: High-speed traces that pass too close to other components or ground planes can experience unintended coupling, impacting impedance. f Solution: Adjust the clearance constraints in Constraint Manager to maintain the proper spacing between high-speed traces and other components or ground planes. Solution Implementation in OrCAD X The Constraint Manager is a powerful tool that allows designers to set and monitor impedance requirements throughout the design. Here's how to set up these constraints to address impedance discontinuities: 1. Setting Up Controlled Impedance: f In Constraint Manager, define trace width, spacing, and layer stack-up settings to achieve the desired impedance level (typically 50Ω for single-ended signals, 90Ω for differential pairs). Specify these rules for all high-speed nets. f The DRC feature will continuously verify that traces meet the defined impedance requirements, flagging any nets that deviate. 2. Defining Via Structures for Layer Transitions: f Assign specific via structures to high-speed nets. By defining via parameters (diameter, pad size, and drill depth), you can limit the impedance impact of layer transitions. f Use the Via Management Tool to select or design via structures that minimize impedance mismatches when transitioning between layers. 3. Enforcing Trace Width Consistency: f Apply trace width constraints to high-speed nets in Constraint Manager to ensure consistent width along the entire path. This is particularly important for differential pairs, where width and spacing must remain constant to preserve impedance. f The tool will flag any deviations, making it easy to correct these issues during routing. Practical Tips for Managing Impedance in High-Speed Designs f ●Avoid Routing Over Split Planes: Routing a high-speed trace over a split ground or power plane introduces significant impedance variations. Keep high-speed traces on continuous planes where possible. f ●Use Short, Wide Traces for Vias: When transitions are unavoidable, use short, wide traces near vias to reduce impedance variation. f ●Simulation and Verification: Run signal integrity simulations to validate the impact of design changes on impedance. The integrated simulation tools allow you to assess reflections and signal quality, enabling pre-layout verification. 2. Signal Reflections, Ringing, and Crosstalk Issues like signal reflections and crosstalk can compromise data integrity and lead to inconsistent performance. Let's explore this phenomena, their causes, and how OrCAD X offers solutions for mitigating their effects. Signal Reflections and Ringing f Problem: Signal reflections occur when a high-speed signal encounters an impedance mismatch along its path. These reflections can cause ringing, which is the oscillation of the signal around its desired level, often manifesting as overshoot or undershoot. This is particularly problematic in high-speed designs, where even minor deviations can lead to data errors and timing issues. 7 www.cadence.com OrCAD X High-Speed Digital Design Guide

• Cover