Issue link: https://iconnect007.uberflip.com/i/1489778
JANUARY 2023 I DESIGN007 MAGAZINE 21 symbol interference, crosstalk, process-volt- age-temperature variations, and other factors amounts to jitter. Some jitter is simply random. e impact of termination is clearly visible in the eye diagrams generated. With improper termination, the eye looks constrained or stressed; with improved termination schemes, the eye becomes more relaxed (open). A poorly terminated signal line also suffers from multiple reflections. A practical way to analyze DDR signals is via the eye diagram. Either read or write DQ is folded into an eye based on the reference clock recovered from the DQS strobe edge. A composite eye diagram can tell the exact jit- ter content and distribution in your memory interface, as the DQ signals are referenced to a clock (strobe signal). Additionally, it can dis- play reflections from incorrect driver selection and on-die-terminations (ODT), as in Figure 4. Variations (reflections) at the peaks on the waveform indicate inappropriate termination. Eye diagrams can also pickup stubs on high- speed serial links. Figure 5 shows the effects of excessively long via stubs on a high-speed dif- ferential pair. On the le, the differential pair is simulated using a pseudo-random bit stream with lossy transmission lines enabled—note the open-eye pattern. However, on the right, I have included via modelling, which enables the via parasitics and highlights the effects of via resonance. e high frequency harmonics Figure 4: DDR3 data streams. 40Ω driver withno ODT (left); 40Ω driver with 80Ω ODT (right). Figure 3: Eye diagram measures signal quality.