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Observer akshay_acharya
Observer
575 Views
Registered: ‎10-11-2018

Eye scan using IBERT

Hello sir,

In the Eye scan obtained using IBERT, The vertical axis refers to voltage codes. So obtained voltage code*2 will provide the actual voltage. But my question is what does the horizontal axis represent? and what is Dwell BER ? How does it affect the eye scan. And what does the colours meant in Eye scan. Please help me with this.

 

Regards,

Akshay acharya

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3 Replies
Moderator
Moderator
544 Views
Registered: ‎07-30-2007

Re: Eye scan using IBERT

We don't typically convert the vertical back to voltage.  We are typically just looking for at least 52 codes of clearance and ideally more that 60 (for UltraScale gt's with a dwell of 10e-9).  The horizontal is the amount of timing margin you have in Unit Intervals (UI).  So if you are at 10Gbps and have .3 clearance that would be a 100ps UI so the .3 would mean 30 ps.  Dwell BER is the number of samples you take for each position of the chart.  DWELL BER of 1e-9 would mean that the eyescan will check for the error rate of 1e-9 at each offset.  To do this it will take 10e9 samples at each offset. For the horizontal we look for .26 to .3 UI of margin.




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Observer akshay_acharya
Observer
498 Views
Registered: ‎10-11-2018

Re: Eye scan using IBERT

So. If I change the dwell BER, the eye diagram will also change?.

I am really confused now. I know what is Unit interval in an eye-diagram. But if I consider the dwell bit error rate and the colour coding, I am not able to relate it to the Unit interval  concept.

and regarding vertical codes to voltage conversion I am not convinced with your answer. I have found somewhere in Xilinx forum that vertical code * 2= voltage. I have attached the neccessary screenshot please do cross-verify weather it is proper or not.

 

Regards,

Akshay Kumar

ibert forum.PNG
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Moderator
Moderator
480 Views
Registered: ‎07-30-2007

Re: Eye scan using IBERT

In general, if you take more samples the eye will get smaller.  The BER dwell lets you set how many samples to take.  Ideally you want to set the dwell to the BER you are interested in acheiving.  Since that can can take too much time the eye mask is done for a BER dwell of 10E-9 and extrapolated to acheive a BER of 10E-15. 

There is a conversion from codes to mv but I'm not sure how accurate it can be after the signal goes through equalization stages and an automatic gain control stage. The conversion is 1.6 mv/code on the GTY.  See the eyescan_vs_range attribute in UG578 (other gt's have similar attributes.) My only point is that since the Eye Scan Mask is in codes that is the more useful measure.  The mask is defined for the eyescan_vs_range setting of 00.

 




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