Printed Circuit Design & Fab - April 2008 - (Page 27) SI OPTIMIZATION Solve Design Problems with SIGNAL INTEGRITY Optimization Optimization routines save time by using automated methods to determine if performance goals are met. by PAT ZABINSKI, BEN BUHROW, BARRY GILBERT, and ERIK DANIEL Printed circuit board (PCB) design often begins with the development of signal integrity (SI) guidelines to ensure adequate performance of the final product. Most often the guidelines are based on traditional engineering practices through manual manipulation of circuit parameters and judicious interpretation of results. While such approaches do result in useful conclusions, they can also consume significant effort, only to reach sub-optimal conclusions. Alternatively, optimization routines can be used to aid in SI analysis and the development of PCB design guidelines. Optimization routines have been well proven to aid analysis across a variety of common tasks, such as determining the optimal values for circuit parameters. In addition, there are several non-traditional applications where optimization can be useful, such as developing application-specific termination schemes. With increasingly higher speeds and routing densities, proper signal integrity (SI) engineering is an important aspect of printed circuit board (PCB) design. At its basic levels, SI engineering involves the development of design guidelines for impedance control, termination, attenuation, and isolation. As design complexity increases, so does the associated SI analysis. Where feasible, common industry practice (i.e., rules of thumb) can guide SI analysis on many issues with good results. In situations where more significant analysis is necessary, SI engineers can potentially benefit from non-traditional approaches. Optimization routines are engineering tools that are often useful in these situations, as they automate the process of tuning parametric values to achieve desired performance. One of the more common applications of optimization routines is the development of equivalent-circuit models1,2 of various passive interconnect. In these cases, the interconnect performance is measured in the lab in either time or frequency domains. Based on the measured performance and knowledge of the physical structure, a circuit topology is generated to approximate the performance. Optimization is then used to adjust the various circuit element values to best approximate the measured performance. In addition to model development, optimization routines are also commonly used for obtaining peak performance of completed links, such as determining optimal termination resistance. Beyond these common applications, there are several nontraditional applications of optimization routines3 that can be of similar value. For example, it is often possible to use optimization routines to assist in developing unique circuit architectures or to optimize cost versus performance trade-offs. Optimization Basics Optimization routines rely on an iterative process similar to that shown in FIGURE 1. For SI analysis, the process generally starts with a circuit model that has undesired simulated performance. Within the circuit model, a specific set of userdefined circuit parameters are selected, and these parameters are allowed to vary within a specified range. The parameters can be typical circuit values such as resistance and capac- FIGURE 1. Flow diagram for generic optimization process. APRIL 2008 FIGURE 2. Multi-drop bus circuit topology. PRINTED CIRCUIT DESIGN & FAB 27
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