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PCBD-Nov2016

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42 The PCB Design Magazine • November 2016 Basically, a differential pair is two comple- mentary transmission lines that transfer equal and opposite signals down their length. We assume that tightly coupled differential pairs have no current in the adjacent planes because the return current of one line is carried by the other. That is not correct. On a PCB, the return current path, of each trace of the pair, flows di- rectly below each trace in the reference plane as seen in Figure 1. If the differential pair is well balanced, then tight coupling will achieve an effective degree of field cancellation. However, if they are not perfectly balanced, then the degree of cancel- lation is not determined by the spacing, but rather by the common-mode balance of the differential pair. Most digital drivers have poor common-mode balance and therefore differen- tial pairs often radiate far more power in the common-mode than in the differential-mode. In such a case, one gains no radiation ben- efit from coupling the differential traces more closely together. According to the FCC Class B compliancy standard, the differential-mode radiation from a microstrip pair, with 20mil separation, should theoretically yield a 40dB radiation improve- ment at 1 GHz over the radiation one would measure from the same signal routed as a single ended trace. It is the common-mode signal that dominates the radiation and decreasing the pair spacing will not improve this situation. For a perfectly balanced differential signal, the radiation from one trace exactly cancels the radiation from the other as they are equal and opposite. However, a common-mode sig- nal represents an average of the two signals in a pair. The radiation is identical on both traces and therefore it does not cancel but rather rein- forces. To minimize radiation and crosstalk you must think explicitly about the common-mode component of the differential signal—skew cre- ates this common-mode signal. Arguably, the principal source of imbalance is time delay skew between the two traces. The easiest way to minimize this skew is to match the electrical lengths and to correct any shift immediately, after it arises, by adding length (hence delay) to the shorter trace. Unfortu - nately, time-delay skew can also be introduced by a variation in the dielectric constant of the glass-resin composite. This, weave induced skew, can be minimized by using materials with a spread-glass weave such as the new Isola I-Speed, I-Speed IS and Tachyon-100G that have been specifically developed for high- speed applications. The transformation from differential to common-mode also takes place on bends and non-symmetrical routing near via and pin ob- structions. In a previous column, Beyond De- sign: Differential Pair Routing, I concluded that symmetry is the key to successfully deploying differential signals in high-speed designs. Main- taining the equal and opposite amplitude and timing relationship is the principle concept when using differential pairs. Mirror symme- try (as in Figure 2) about an axis, along the in- terconnects, avoids mode transformation. The symmetry property preserves the signal in the differential-mode which does not radiate. Com- mon-mode noise may have little effect on sig- nal integrity, but will have a more serious im- pact for EMI. Mode transformation can also be minimized by reducing the size of any bends in the pair. Any skew introduced, by a bend, should be cor- rected immediately after the bend so that the majority of the length of the pair is balanced. Also, routing in a stripline configuration has UNCOMMON SENSE Figure 1: Return current paths of a differential pair (courtesy Ansoft).

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