Measuring the Performance of Equalized Serial Data Links Across the Design Flow

White Paper
MEASURING THE PERFORMANCE OF EQUALIZED
SERIAL DATA LINKS ACROSS THE DESIGN FLOWMarch 2009
ABSTRACTFor advanced signaling over high-loss channels, designs today are using equalization and severalnew measurement methods to evaluate the performance of the link. Both simulation and measurementtools support equalization and the new measurement methods, but correlation of results throughoutthe design flow is unclear. In this paper a high performance equalizing serial data link is measured andthe performance is compared to that predicted by simulation. Then, the differences between simulationand measurements are discussed as well as methods to correlate the two.
Authors:Steven McKinney (Steven_McKinney@mentor.com)Vladimir Dmitriev-Zdorov (Vladimir_Dmitriev-Zdorov@mentor.com)Mentor Graphics Corporation8005 SW Boeckman RoadWilsonville, OR 97070 USA Phone: +1 800-592-2210 or +1 503-685-7000
and
Pavel Zivny (pavel.zivny@tek.com)Maria Agoston (maria.agoston@tek.com)John Carson (john.e.carlson@tek.com)Tektronix Inc.14200 SW Karl Braun DriveBeaverton, OR 97077Phone: +1 800-835-9433
www.mentor.com/pcbSuccessful high speed serial interconnect design generations. New tools complemente jitter, noise andrequires thorough simulation of the link, and BER analysis with models of transmitter, channel, andconsequent measurement verification and receiver elements. These tools will answer questionscharacterization of the implemented design. A lossy like: Will the legacy link stay compliant as data rateschannel can have more than 30 dB loss at the first climb from 3 to 6 Gbps? Will the system still meetharmonic, and have an eye diagram that is well past compliance when cable length growth from 6 to 10 m? complete closure at the receiver end of the link. Forsignaling over these lossy channels, designs today are Measurement-based emulation tools provide meansusing equalization in both the transmitter and in the for answering questions by providing emulation ofreceiver. Older, simple measurement methods, such as the full communication link with configurable ele-horizontal eye opening at bit error rate (BER), or a ments that allow for adding or removing transmittermask test completely fail on closed eyes and do not emphasis, probe de-embedding, channel emulation,provide performance metric. as well as equalization tools for testing thereceiver capabilities.New measurement methods share origins with the firststandard-ratified measurement method for heavily A system-level workflow model describes the designequalized links, the IEEE 802.3aq 10GBASE-LRM. and deployment phases for components of high speedThis evaluation method for transmitter performance, serial links, from SERDES chips to cables, back-the TWDP, Transmitter Waveform Dispersion Penalty, planes, etc. Figure 1 shows the role of the simulationuses channel simulation, equalizer simulation, and tools in the design phase of devices, migration tomeasures a form of signal-to-noise ratio on the correl- prototype turn-on, verification, characterization andated data. TWDP started in the optical domain, and the finally, manufacturing, measurement-based stages ofmethod uses three corner cases of how a multimode analysis. Between characterization and manufacturing,fiber can impact the signal, with the so-called pre- there is the certification or compliance test step.cursor, symmetrical, and postcursor response.
The development of new measure-ment methods is tracked by bothsimulation and measurement toolssupporting equalization and chan-nel insertion. This paper comparesmeasurement methods and focuseson correlated eye closure (eyeclosure by the ISI of the channel).
ROLE OF SIMULATION ANDMEASUREMENT-BASEDEMULATION Serial data communication linksare targeting higher data rates bymoving into second and third Figure 1: Design and Deployment workflow
www.mentor.com/pcb 1Given this background, this paper will demonstrate finally, Figure 4 shows a setup that allows forthat, given an equalized 6.25 Gbs serial data link: measurements at the transmitter, and by emulating a? This serial data link can be simulated with Mentor channel and an equalizer, estimate the BER at theGraphics simulation tools. receiver side. ? The implemented design can be verified andcharacterized with Tektronix measurement tools.? Simulation can predict worst-case pattern behavior.? There is a good match between performanceexpected by simulation, measu... [download for more]