smlogan
Community Member
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Posts: 52
Boston, MA
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Hi Raj,
... it looks top eye diagram looks very clean and i would expect less jitter compared to the bottom, their Quality looks very far that's why i could comment on that. But measurements shows exactly opp conclusions in the slide 33&34. can any one point me some points regarding this??
A good question indeed! However, the conclusion drawn by the article is likely correct. The reason that the eye diagram on the bottom of slide 32, which appears to have a larger peak-to-peak jitter than the eye diagram on the top, is said to be superior lies in the measurement methodology and, ultimately, the type of jitter. If you examine slide 33 - which decomposes the jitter of the lower eye diagram on slide 32 - the random jitter component is measured as 2 ps rms and the deterministic components account for the remainder. Slide 34 - which decomposes the jitter of the upper waveform of side 32 - shows a random jitter component of 11 ps rms. In many applications where bit error rate is a concern, the peak-to-peak random jitter component is estimated for a BER of 1e-12 as 14*sigma. Hence, with a 2 ps rms random component, the peak-to-peak random jitter in a 1e-12 BER application is 14*2 ps rms = 28 ps. However, in the upper waveform on slide 32, the random jitter contribution in a 1e-12 BER application is 14*11 ps rms = 154 ps. Hence, the actual eye closure when observed over a significant period of time of the upper waveform in slide 32 is much greater than that shown in slide 32.
If the measurements shown in slide 32 were taken for a greater period of time, the eye closure of the top waveform would indeed show greater eye closure than that of the lower waveform. The fact that the measured data was taken over a relatively short period of time leads to the deceptive eye diagrams. Moreover, the short time period over which the measurements were taken probably contribute to a poor estimate of the random jitter standard deviation. Hence, a better overall assessment, in my opinion, is to capture both waveforms for a longer time period.
I hope this makes sense. Let me know if I have confused the issue.
Shawn
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