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ing systems, design changes, manufacturing process changes, or both should be made for the next product generation to reduce returns and warranty claims. Hiroshi Goto, business development manager at Anritsu Co, suggests pre- emphasis as an effective transmission technique for maintaining the eye opening. With transmission speeds increasing to 20 Gbits/sec and faster, Goto proposes a three- or four-tap emphasis signal in order to increase the number of bits to be emphasized. It’s a complex job to check and set the combination of emphasis rates for each tap, however, making it difficult to find the ideal emphasis signal without quantitative guidelines. The Anritsu-developed MP1825B four-tap emphasis and transmissionanalysis software, working with the MP1800A signal-quality analyzer BER test set (BERTS), finds “the ideal emphasis settings based upon the reverse characteristics” of the de- vice under test (DUT), says Goto (Figure 3). “This raises the height of the eye and keeps the eye open, allowing better quantitative signal-integrity analysis in the shortest amount of time.” Simulation and validation Most agree that simulation is becoming mandatory for high-speed system design. Agilent’s Howard says the company’s Advanced Design System (ADS) is the leading EDA software in use for Figure 3 The Anritsu MP1800A 32G synchronized multi-BERTS and MP1825B 28.1G four-tap emphasis aim to assist signal-integrity analysis by keeping the eye open. high-speed digital applications. Teledyne LeCroy’s Blankman adds that to detect and mitigate crosstalk issues, designers must be able to predict near- and far-end crosstalk by running simulations, and to validate the models used in the simulations by taking measurements (Figure 4). To validate crosstalk models, designers need multidifferential lane S-parameter measurements (eight-port for aggressor-victim models, 12-port for aggressor-victimaggressor models, or even higher port counts). Measuring crosstalk requires vertical noise measurements taken by real-time oscilloscopes that can extract the crosstalk from the serial data signal. Those measurements should estimate eye closure as a function of BER, as jitter measurements do. Jitter measurements are also important, of course. Measuring both jitter and noise yields a more complete picture of crosstalk than jitter measurements alone. Toolbox Test-equipment vendors are working to evolve their tools to characterize jitter and improve signal-integrity analysis, so the optimal toolbox for signal-integrity engineers may not yet be available. Signal Consulting’s Johnson predicts that “the next trend will involve a blend of specialized equipment and test software designed to characterize a power system and inject specific test current waveforms into that power system.” Stephens, of Ransom’s Notes, suggests that we be on the lookout for more crosstalk-equalization techniques. So, what’s out there now? • Scopes. Here is where high-bandwidth oscilloscopes can really prove their worth. Teledyne LeCroy’s Blankman notes that nonreturn-to-zero (NRZ) serial data patterns can have rise times less than 30 psec. He points out that receiver testing of PCIe Gen3 systems requires a Figure 2 The Agilent U4154A logic analyzer uses its eye-scan capability to place the sampling point automatically in both time and voltage within the eye, making measurements on eye openings as small as 100 psec × 100 mV. Figure 4 The SPARQ signal-integrity network analyzers from Teledyne LeCroy connect directly to the DUT and PC-based software through a single USB connection for quick, multiport S-parameter measurements. 20 EDN Europe | MARCH 2013 www.edn-europe.com


EDNE MARCH 2013
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