EDNE September 2012 - (Page 16)

A mo insight intonthly T&M techn the latest o the pages logy from Measurem of Test & (www.tmw ent World orld.com ) ® Test & Measurement World is the leading monthly magazine for engineers and managers in the electronics testing industry. Crosstalk problems are back Cr ossta l k , a p r o b le m a s s o C i at e d wi th pa ra llel buses, i s now a n i ssue wi th h ig h - sp e e d se r i a l bu s e s, w he r e m u lti ple si g na ls o n a si ng le boa rd Ca n i nterfere w ith e aC h ot he r . By Ransom Stephens, Contributing Technical Editor, Test & Measurement World ® 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 40Gbps 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 interferencecanceling differential signaling and jitter-canceling 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 channel 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, closedeye diagrams, and pre-emphasis and de-emphasis, 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 seFigure 1. An eye diagram has no crosstalk will have no impairments. Courtesy of Tektronix. rial 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.” What is crosstalk, anyWay? Crosstalk is the electromagnetic interference of multiple signals 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 psec at 10 Gbps or 5–10 psec at 25 Gbps, the emerging high-rate standards have rapidly changing electric fields that can couple from one lane to another. The faster the change, the louder the crosstalk. High-speed serial systems use differential signaling to beat back all types of EMI (electromagnetic interference). Differential pairs consist of two conductors. One carries the signal, and the other carries the inverse of the signal, or its complement. 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. Photos courtesy of tektronix. 16 EDN EUROPE | SEPTEMBER 2012 www.edn-europe.com http://www.tmworld.com http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE September 2012

Cover
Contents
International Rectifier
Microchip
RS Components
Masthead
Microchip
EDN Comment
Pulse
Analog Devices
Altera
Baker's best
Messe München
Test & Measurement World
Agilent Techno
Digi-key
Bergquist
Advanced power switches boost microhybrid emissions gains
Digi-Key
Image sensors evolve to address Emerging embedded- vision needs
Renesas
Silicon Labs
Digi-Key
Vicor
Power becomes a software issue as smart phones become smarter
IAN
Power : a significant challenge in EDA design
Digi-Key
Mechatronics in Desin
Design Ideas
Product roundup
Tales from the Cube

EDNE September 2012