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May 2017 • The PCB Design Magazine 45 The easiest way to reduce crosstalk, from a nearby aggressor signal, is of course to in- crease the spacing between the signals in ques- tion. Doubling the spacing cuts the crosstalk to roughly a quarter of its original level. How- ever, crosstalk is determined by the ratio of the trace separation and also the height of the trace above the plane. By varying the trace height, one can also control the coupling–hence cross- talk. If real estate is limited, then this may be a better solution rather than increasing routing density. A tight coupling (less height) results in less crosstalk. The return current distribution of two par- allel traces (Figure 5) shows an overlap of cur- rent in the surface of a microstrip plane. In fact, the overlap will be larger at lower frequencies where the return currents tend to spread out and not follow a tight path under the trace. Also, as the voltage increases so does the cou- pled noise. This is a good reason not to inter- mingle dissimilar technologies but rather keep them isolated. If we look into it further, the degree of crosstalk is also dependent on several oth- er factors including driver strength (which can normally be adjusted in the firmware), transmission line length, how far the seg- ments run closely in parallel and signal rise time. In the case of long transmission line lengths, a series terminator slows the signal rise/fall time and reduces reverse-coupled crosstalk at the near end improving crosstalk considerably. With all of these issues, to take into ac- count, how do we ever get high-speed transmis- sion lines, particularly those with wide parallel buses, to work efficiently? Fortunately, synchro- nous buses, as typically used in DDRx designs, benefit from an extraordinary immunity to crosstalk. Crosstalk only occurs when the sig- nals are being switched and this crosstalk only has an affect within a small window around the moment of clocking. So, providing the receiver waits sufficiently long enough for the crosstalk to settle before sampling the bus, the crosstalk has no impact on the signal quality at the re- ceiver. It's all about timing—ensuring that the required setup and hold times are provided at the receiver. Remember: "Beware the dark side. Anger, fear, aggression; the dark side of the Force are they. Easily they flow, quick to join in a fight. If once you start down the dark path, forever will it domi- nate your destiny, consume you it will." —Yoda Points to Remember • Current must return to the source to complete the loop. • Returning signal currents tend to stay in close proximity to their signal conductors, falling off in intensity with the square of increasing distance. THE DARK SIDE – RETURN OF THE SIGNAL Figure 4: Coupling levels off above 12 mils sepa- ration (simulated by the iCD Stackup Planner). Figure 5: Parallel traces and return path crosstalk.

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