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30 DESIGN007 MAGAZINE I JUNE 2024 e characteristic impedance of a transmis- sion refers to the impedance seen by a driver looking down an infinitely long trace. Interest- ingly, the characteristic impedance is indepen- dent of trace length. It's a measure of the ratio of inductance to capacitance at any point along the trace. erefore, while trace length matters for signal propagation delay, it doesn't directly impact characteristic impedance. When a transmission line is perfectly matched to the Termination Planning driver and load, the signals propagating elec- tromagnetic (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 reflections do occur whenever the impedance of the transmission line changes along its length. Mismatched impedance causes signals to reflect back and forth along the lines, which causes ringing at the load (Figure 1). e ringing reduces the Beyond Design Feature Column by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Reflections cause over/undershoot and jitter of the signal eye.

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