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Original question:

DS250DF410: ds250DF410 Eval Board PRBS Generator

DS250DF410: ds250DF410 Eval Board PRBS Generator,how to solve eye out undershoot?

kailin zhao
Intellectual 260 points
Part Number: DS250DF410
Other Parts Discussed in Thread: DS280DF810

Dear e2e

I used DS250DF410 as retimer,Input a PRBS31,25.78Gbps signal to RX0,and get a output in TX0,but the eye diagram output of TX0 is undershoot,when I set  post and pre cursor ,the undershoot is missed,but the 

jitter is increase.  I want to get a low jitter and no undershoot out,what should I do? There is the two eye disgram.

thanks

  • 0
    TI__Guru* 93670 points
    Hi Kailin,

    1). Are you saying the 2nd waveform - precursor = -4 - has undershoot and jitter is high? The purpose of the pre-cursor is to emphasize high frequency and de-emphasize low frequency content. So you would see this undershoot if you make this measurement right at the output of the device. It would be a good idea if you can make this measurement at the end of your transmission media with pre-cursor = -1 and then compare with another waveform where you set pre-cursor to -4. These measurements will help to optimize pre and post cursor settings.

    2). Please below note my email address. I would appreciate if you could please let me know about your project application and time line(production date and etc). Here is my email address: Nasser.mohammadi@ti.com

    Regards,,nasser
  • 0 in reply to Nasser Mohammadi
    Intellectual 260 points

    Hi nasser

    thanks for you answer.I follow your step.There is the test data bellow.

    when I set Pre Cursor Tap=-1,Post Cursor  Tap=-1,Main Cursor=26,I get a Lower Jitter-RMS and undershoot eye diagram.

    when I set Pre Cursor Tap=-1,Post Cursor  Tap=-4,Main Cursor=26,I get a Lager  Jitter-RMS and lower undershoot eye diagram.

    when I set Pre Cursor Tap=-4,Post Cursor  Tap=-4,Main Cursor=23,I get a Lagest Jitter-RMS and no undershoot eye diagram.

    I would like to get a Lowest Jitter and no undershoot eye diagram,what should I do?  Thank you!

  • 0 in reply to kailin zhao
    TI__Mastermind 43795 points

    Hi Kailin,

    Have you tried using Pre-cursor = 0?

    Have you tried using Post-cursor settings of -2, -3, or -5?

    Each setup is a little different to get the optimal results you may have to move the values one step at a time.  In our lab we often find the lowest jitter has a slight "undershoot" as you are describing in the eye diagram.

    Regards,

    Lee 

  • 0 in reply to Lee Sledjeski
    Intellectual 260 points
    Hi Lee
    I tried many group of different pre cursor,post cursor,main cursor and I could not fine the group that get lowest jitter and slight undershoot.
    pre cursor from -8~+8,post cursor from -8~+8,main cursor from -26~+26.But I still not find the group.

    thank you
  • 0 in reply to kailin zhao
    TI__Mastermind 43795 points

    Hi Kailin,

    These eye openings are still quite large in amplitude.  In some cases lowering the Main cursor amplitude can produce the same low jitter waveform with less overall "undershoot" as you describe.  In most cases I would recommend to utilize the solution with a little undershoot and low jitter as most receivers will benefit from this combination.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Intellectual 260 points
    Hi Lee

    I tried reduce the amplitude,the undershoot is missing,but the jitter is increase.such as,I set the main cursor is +26,the pre cursor is -1,post cursor is -1,I get a 1.291ps jitter-RMS,when I set main cursor +12,the pre cursor is -1,post cursor is -1,I get a 1.535ps Jitter-RMS.

    thank you
  • 0 in reply to kailin zhao
    TI__Mastermind 43795 points

    Hi,

    Given all of the adjustments you have made, I would suggest that you select a compromise setting.  One that can balance out the need for low Rj and as little undershoot as possible.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Intellectual 260 points
    Hi Lee

    I test some cases,maybe the reson of undershoot is less bandwidth of DS250DF410, so I prepare to use DS280DF810 for my design.
    How to request the detail datasheet of DS280DF810?

    Thank you
  • 0 in reply to kailin zhao
    TI__Guru* 93670 points

    Hi Kailin,

    I am not sure if going to DS280DF810 could resolve the undershoot since this is due to the output driver and impedance of the media that you are using. Please below note a link for requesting access to the DS280DF810 design documents. 

    https://www.ti.com/drr/opn/DS280DF810-DESIGN

    Regards,,nasser