Calling out crosstalk

The 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 Express capitalize on the robust nature of serial technology, with its interference-canceling differential signaling and jitter-canceling embedded clocking. To achieve their data rates, these buses employ multiple serial lanes that operate in parallel. With each additional lane, a bus scales to a higher data rate (Ref. 1).

Unfortunately, every lane 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-emphasis, and then having equalization improve signal quality and reduce bit errors, 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 interconnects, 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."

Go directly to the article on Test&Measurement World. It covers, what is crosstalk? how scopes analyze crosstalk, and looks specifically at how Tektronix, Agilent, and LeCroy have addressed it.