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DDR3 DQ quality questions

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fightforever

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Hello Everyone,Recently I am debugging the DDR3 SDRAM controller, and get some waveforms which probed by TEK 50Gsps Oscilliscope(Fig.1)
In Fig.1(up) Eye Diagram,the orange eye is DQS, green eye is DQ; In Fig.1(down), the yellow line is DQS,the purple line is DQ.
My question is about the DQ signal: why does it go obviously slow when it reaches 926mv(position -0.5ns eye diagram)? why not the waveform like the following picture(fig.2)?
does the power cause this?
the DQ line trace is like the Fig.3
35_1271512387.jpg
 

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