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PCBD-July2016

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28 The PCB Design Magazine • July 2016 for minimum intersymbol interference typical- ly yields a slightly greater eye height. Figure 10 is an example of the trade-off that can be expected for the case of a single tap FFE and a channel that is relatively easy to equal- ize. The green line is the average amplitude in the eye diagram at the receiver decision point, which is proportional to the main tap weight. The blue curve is the maximum intersymbol in - terference at the recovered clock time and the red curve is the minimum eye height at the re- covered clock time. The red curve is equal to the green curve minus the blue curve (i.e., the aver- age amplitude minus the intersymbol interfer- ence). In this case, reducing Tap 1 by a little over 4% increases the eye height by a little over 1%. While more sophisticated algorithms are pos- sible, for manual optimization it should be suf- ficient to reduce all of the equalizing tap weights in the FFE by a fixed percentage and add that amplitude to the main tap. It would be practical to evaluate the eye height for several different values of the fixed percentage; however, in most cases a value from 0% for channels that are diffi- cult to equalize to about 5% for channels that are easy to equalize should be about right. PCBDESIGN Due to the length of this article, we have pub- lished the remainder on the PCBDesign007 site. Click here. Donald Telian is an independent signal integrity consultant with Si- Guys. Building on over 30 years of SI experience at Intel, Cadence, HP, and others, his recent focus has been on helping customers correctly imple- ment today's multi-gigabit serial links. To contact him, click here. Michael Steinberger, Ph.D., lead architect for SiSoft, has over 30 years of experience designing very high-speed electronic circuits. He is currently responsible for the archi- tecture of SiSoft's Quantum Channel Designer tool for high speed serial channel analy- sis. To contact him, click here. Barry Katz, president and CTO of SiSoft, founded the company in 1995. As CTO, Barry is responsible for leading the definition and devel- opment of SiSoft's products. He has devoted much of his efforts at SiSoft to delivering a comprehensive design methodolo- gy, software tools, and expert consulting to solve the problems faced by designers of leading edge high-speed systems. To contact him, click here. Figure 10: Trade-off between intersymbol interference and eye height for a single tap FFE. NEW SI TECHNIQUES FOR LARGE SYSTEM PERFORMANCE TUNING

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