Issue link: https://iconnect007.uberflip.com/i/1539283
28 DESIGN007 MAGAZINE I SEPTEMBER 2025 3. Multiload Scenarios For buses with multiple receivers, termination becomes more complex. In these cases, series ter- mination alone is often insufficient, as reflections from each load can still affect other loads; parallel termination at the end of the bus can be effective if the stubs to each load are kept very short; and careful layout is critical to minimize impedance discontinuities. Another Signal Integrity Challenge: Crosstalk While proper termination addresses reflections, another significant signal integrity concern is crosstalk. This is unwanted coupling of energy between adjacent signal traces. Crosstalk occurs through both capacitive and inductive coupling between traces. There are two types of crosstalk: • Near-end crosstalk (NEXT): Measured at the same end as the aggressor signal source • Far-end crosstalk (FEXT): Measured at the opposite end from the aggressor signal source To minimize crosstalk: • Increase the spacing between traces • Decrease the coupling length • Use reference (ground) plane pours in between (separating) critical signals on the same signal layer where possible, as well as implementing reference planes in between adjacent signal layers within the PCB stackup • Consider the dielectric thickness and trace geometry in stripline configurations of the PCB stackup (where the trace is embedded between ground planes) Advanced Analysis Tools EDA tools: These software tools simulate and analyze the electrical characteristics of high-speed designs and are often integrated into the PCB design flow. These advanced SI analysis tools include software suites that provide things like 3D field solvers, impedance and crosstalk analysis, and IBIS AMI modeling. Hardware tools: Tools like high-performance os- cilloscopes and vector network analyzers (VNAs) are also crucial for time-domain (eye diagrams and frequency-domain (S-parameters) analysis to validate designs and diagnose issues. Time domain reflectometry (TDR) is a powerful technique for analyzing transmission line disconti- nuities. By sending a fast edge into a transmission line and measuring the reflections, engineers can identify and locate impedance discontinuities. For example, TDR can be used to analyze via performance. A typical via might show a capacitive discontinuity followed by an inductive discontinu- ity. The TDR plot would show a dip in impedance at the via location, while frequency domain analy- sis might reveal increased insertion loss and return loss at higher frequencies. Let's break this down to expound on the frequency-domain analysis of an interconnect, and how it shows losses and resonances. TDR is like taking an X-ray of your circuit board. It shows you exactly where the "bumps" and "dips" in impedance are, like that via example where it shows a little dip then maybe a bump. It's great for pinpointing "where" issues are. Now, "frequency domain analysis" is a different beast. Think of it less like an X-ray and more like a full health report on your signal, telling you how it performs at "different speeds" (that's what "fre- quency" means here—from slow to super-fast). We use VNA equipment to get these reports, and the main things we look at are called S-parameters. These are a frequency-domain representation of a network that characterizes the electrical behavior of the network at its external ports. Multiple resources on the web provide details of defining, measuring, and using S parameters. In the via example, TDR gives you a hint ("dip in impedance at the via location"). But frequency domain analysis really spells it out: "At these