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86 I-CONNECT007 MAGAZINE I MAY 2026 Figure 2: Unmatched (red) vs. matched (blue) transmission lines. Unfortunately, drivers do not have the same im- pedance as the transmission line (typically 10–35 Ω), so series terminations are used to balance the impedance, match the line, and minimize reflec- tions, particularly on long traces where on-die termination is not provided. Impedance matching slows down the rise and fall times, reduces the ring- ing (over-/undershoot) of signal drivers, and enhanc- es the quality of a high-speed signal. The ringing is dramatically reduced by adding a series terminator as in Figure 2. From this, we can see that the imped- ance has to be matched, but to what value? In Figure 3, using a 12 mA LVCMOS 1.8V driver of a Spartan 6 FPGA, an 18.7 Ω series resistor is required to match the driver to the 51.67 Ω trace on the outer layer. This is automatically derived from the IV curves of the Spartan 6 IBIS model by the iCD Termination Planner. When a signal's electromagnetic energy propa- gates from the driver to the receiver along a transmission line, it changes along its length. The original signal will be received with varying de- grees of distortion and degradation. This signal distortion happens due to factors such as imped- ance mismatch, reflections, ringing, crosstalk, dielectric loss, jitter, and ground bounce. The PCB designer's primary objective should be to minimize these issues at the source, so that any signal dis- tortion is eliminated. But unfortunately, even with perfect impedance and termination, a signal can still be corrupted if its return current is forced to take a detour. Another culprit is crosstalk, particularly on long parallel trace segments. Crosstalk arises as a result of the unintentional coupling of electromagnetic fields and causes both forward and reverse reflec- tions. The easiest way to reduce crosstalk from a nearby aggressor signal is, of course, by increas- ing the spacing between the signals in question. Crosstalk falls off very rapidly with distance, plum- meting roughly quadratically with increased sepa- ration. Doubling the spacing cuts the crosstalk to roughly a quarter of its original level. A good rule of thumb for this is Gap = 3x trace width. However, in today's complex designs, it is not always pos- sible to use up valuable real estate to satisfy the B E YO N D D ES I G N