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22 DESIGN007 MAGAZINE I MAY 2022 When a transmission line is perfectly matched to the driver and load, the signals propagating electromagnetic (EM) energy are totally absorbed by the load. is is the perfect scenario that all electronics designers strive for. However, this is rarely the case and reflec- tions do occur whenever the impedance of the transmission line changes along its length. is can be caused by unmatched drivers/loads, layer transitions, different dielectric materials, stubs, vias, connectors, and IC packages. By understanding the causes of these reflections and eliminating the source of the mismatch, a design can be engineered to perform reliably. ere are many ways to dampen reflections in high-speed PCB design, as listed below: Drive Strength Reflections can be caused by using a drive strength that is too high for the load. Driver strengths typically range from 4 mA to 16 mA. A 16 mA driver is generally required for mul- tiple loads—for instance, a DDR4 signal driv- ing multiple SODIMM memory cards. In this case, the transmission lines are longer and the capacitive load higher, so simulation is neces- sary to confirm the required driver's current strength. To dampen the signal, terminations are typically placed on the memory card itself. However, if the signal is delivered to only one or two on-board memory devices, then the sig- nal strength can usually be reduced to a mini- mum of 8 mA to prevent reflections. e simulation in Figure 1 shows a clean signal with a wide eye opening, but there is a slight over-and undershoot of the signal (top and bottom) and a small amount of timing jitter (horizontal misalignment). e over/ undershoot can be attributed to either over- driving the transmission line using a higher Reflecting on Reflections Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Reflections cause over/undershoot when the transmission line is over-driven.

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