+ Post New Thread
Results 1 to 5 of 5
  1. #1
    Full Member level 3
    Points: 2,739, Level: 12
    gilbertomaldito's Avatar
    Join Date
    Aug 2006
    Posts
    159
    Helped
    10 / 10
    Points
    2,739
    Level
    12

    Weird Eye jitter. Can anybody explain why?

    Hi,

    Ok, I simulated my LVDS receiver using input K28.5 pattern at 1Gbps. The eye diagram shown below is the eye pattern at the output. My question is, why is it that the waveforms that constitutes the eye are grouped into separate strands?

    The data pattern i used is K28.5 or 0011111010110000010...

    --andrew

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 39,706, Level: 48

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,413
    Helped
    1872 / 1872
    Points
    39,706
    Level
    48

    Weird Eye jitter. Can anybody explain why?

    That seems to me, the classic form of intersymbol interference.
    Is your driven line, longer than 1/2 electrical time-of-flight?



    •   AltAdvertisement

        
       

  3. #3
    Full Member level 3
    Points: 2,739, Level: 12
    gilbertomaldito's Avatar
    Join Date
    Aug 2006
    Posts
    159
    Helped
    10 / 10
    Points
    2,739
    Level
    12

    Weird Eye jitter. Can anybody explain why?

    Hi dick,

    What do you mean by electrical time-of-flight?



    •   AltAdvertisement

        
       

  4. #4
    Advanced Member level 5
    Points: 39,706, Level: 48

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,413
    Helped
    1872 / 1872
    Points
    39,706
    Level
    48

    Weird Eye jitter. Can anybody explain why?

    Wire lines propagate signal at about 1nS/foot. To null a transmission
    line after a signal step requires a wavefront round-trip (with near-
    perfect termination). If you have "one in the pipe" and launch a
    second transition, the "symbols" will collide and add / oppose at
    the receiver, modulating the "eye" edges.

    I'd say at 1GBPS you would be likely to see this at any length
    over 6" or so. Most line interface parts for high speed and long
    wire have some sort of pre-emphasis and try to adjust it
    based on signal history, or so I gather, to correct that effect.



  5. #5
    Full Member level 1
    Points: 1,148, Level: 7

    Join Date
    May 2010
    Posts
    106
    Helped
    5 / 5
    Points
    1,148
    Level
    7

    Weird Eye jitter. Can anybody explain why?

    Yes, case of data dependent jitter. I can't figure out why low to high has the more classic form whereas the high to low has 3 strands.

    btw, there can be other things causing DDJ other than jsut board trace. It'll be good if the OP Can specify exactly what his setup looks like



--[[ ]]--