Test & Measurement World, July/August 2012

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OSCILLOSCOPES problems are back Crosstalk Crosstalk, a problem associated with parallel buses, is now an issue with high-speed serial buses, where multiple signals on a single board can interfere with each other. BY RANSOM STEPHENS, CONTRIBUTING TECHNICAL EDITOR T he inexorable demand for electronic systems with increasing bandwidth and decreasing size puts more high-speed channels in ever-closer proximity. Technologies such as 40-Gbps and 100-Gbps Ethernet employ up to 10 channels at 10 Gbps each or four channels at 25 Gbps. When so many high-speed serial lanes reside in a single system, they're bound to interfere with each other. Serial buses such as Ethernet, Fibre Channel, and PCI Ex- press capitalize on the robust nature of serial technology, with its interference-canceling differential signaling and jitter-can- celing embedded clocking. To achieve incrementally greater data rates, the emerging technologies employ multiple serial lanes that operate in parallel. With each additional lane, a bus scales to a higher data rate (Ref. 1). Unfortunately, every chan- nel is both an aggressor and a victim. Differential signaling can only partially cancel crosstalk at these high data rates. After a decade spent developing serial data technologies and dealing with jitter, closed-eye diagrams, and pre-emphasis and de-em- phasis, and then having equalization save the day, engineers have realized that crosstalk has come back to haunt them. Marty Miller, chief scientist at LeCroy, put it like this: "When we started switching from parallel to serial intercon- nects, crosstalk stopped being a big concern, but now we're moving to serial channels in parallel. We have systems with dozens of SerDes on one chip operating at 50 times the data rate [of parallel buses]. We're looking at a crosstalk nightmare. " What is crosstalk, anyway? Crosstalk is the electromagnetic interference of multiple sig- nals that occurs when radiation from an aggressor channel is picked up by a victim channel. Maxwell's equations describe electromagnetic radiation only when electric and magnetic fields change. Crosstalk is generated during logic transitions. With rise and fall times of 20–30 ps at 10 Gbps or 5–10 ps at 25 Gbps, the emerging high-rate standards have rapidly changing electric fields that can couple from one lane to an- other. The faster the change, the louder the crosstalk. High-speed serial systems use differential signaling to beat back all types of EMI (electromagnetic interference). Differ- ential pairs consist of two conductors. One carries the signal, and the other carries the inverse of the signal, or its comple- ment. If the conductors are arbitrarily close together and are of precisely the same length, then they carry identical EMI. The receiver takes the difference of the pair, canceling the interference and reinforcing the differential signals. This is called common-mode rejection, because signals common to both elements of the pair are rejected. Unfortunately, real differential pairs have non-zero separa- tion, are not exactly the same length, and suffer other asym- metries such as impedance variations at contact points and variations in trace widths, thickness, and roughness. These im- perfections limit common-mode rejection. Engineers developing multilane serial links anticipated cross- talk's return, but that's not the whole story. Pavel Zivny, domain expert on high-speed serial data with Tektronix, said, "Intersig- nal crosstalk was expected, but there's another source of cross- talk that we didn't anticipate. As the transmitters on a SerDes chip draw current from the power plane, they generate power- supply variations that have different properties." Current draw Test & Measurement World | JULY 2012 | –16–

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