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Design007-Nov2023

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20 DESIGN007 MAGAZINE I NOVEMBER 2023 Modelling Single-ended and Differential Via Modelling vias is one of the most frequently searched terms driving traffic to the Polar web- site. One of the counterintuitive points on via modelling is that it is easier to obtain an estimation of a differential via's characteris- tics than that of a single-ended via. Why? Because differential vias are inherently an out-and- return path, there are no discon- tinuities in the return path (or at least they are minor) compared with single-ended vias. is is a key advantage for designs with ultra high-speed digital signaling in making them easier to design. While you can model a single-ended via with a 3D solver, it only goes to prove the design is compromised. To get the best signal integrity for a single-ended via, it can be necessary to place it next to a ground via which spans all the intermediate planes. Plane pairs to single-ended vias act as slot antennae and cause reflections out and back to the edge of the board. Transitioning a signal from L1 to L3 with a ground on L2 would minimize this, but at same time, you have to check that the stub is still short enough not to be troublesome. Communication Once your stackup design is completed, then ensure it is documented and make it clear to your fabricator, broker, or shop floor that you are a PCB apps engineer so that drills/back- drills, layer separation, and any transmission lines are clearly identified and associated with the relevant net classes. Conclusion Keeping high speed line length as short as possible, stubs short, and careful attention to via interconnects can ease your signal integrity challenges at minimum cost. Use simple tools to flag areas where more detailed modelling or more advanced materials may prove necessary and remember that in most cases engineering is about delivering to your employer and cus- tomer the most appropriate solution for the specific application at the lowest cost. Find more information about signal integrity on our website and YouTube channel. DESIGN007 Martyn Gaudion is managing director of Polar Instruments Ltd. To read past columns, click here. Martyn is the author of The Printed Circuit Designer's Guide to… Secrets of High-Speed PCBs, Parts 1 and 2. Figure 5: Stackup with microvia/back-drilled vias and buried vias. (Source: Polar Speedstack) Figure 4: Differential via property calculation, Polar Si9000e. (Courtesy: Bert Simonovich Lamsim Enterprises)

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