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

kailin zhao

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

over 7 years ago

0 Nasser Mohammadi over 7 years ago

TI__Guru* 93570 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 kailin zhao over 7 years ago 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 Lee Sledjeski over 7 years ago 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 kailin zhao over 7 years ago 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 Lee Sledjeski over 7 years ago 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 kailin zhao over 7 years ago 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 Lee Sledjeski over 7 years ago 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 kailin zhao over 7 years ago 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 Nasser Mohammadi over 7 years ago in reply to kailin zhao

TI__Guru* 93570 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

Interface

Interface forum

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