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Question about jitter budget of TLk3101

David Zhang16358
Expert 1860 points

I have used application note SLLA071 as reference for my jitter budget plan.

The Optical Transceiver we are using is AFBR-59R5LZ, the transmitter side of AFBR-59R5LZ contributed 0.26UI jitter and receiver side of AFBR-59R5LZ contributed another 0.26UI jitter.

 

The transmitter side of TLK3101 contributed max 0.2UI output jitter.

 

Based on my understanding, the sum of above three jitters 0.26UI + 0.26UI + 0.2UI = 0.72UI should be less than 0.6UI, which is the jitter input tolerance of TLK3101, since all those above three jitters decided the input jitter to TLK3101.

 

But on SLLA071, the 0.2UI output jitter plus 0.6UI input jitter tolerance has been used to decide the media jitter requirements.

 

Please answer me.

 

Thanks in advance.

  • 0
    Expert 1860 points

    Any answers from you professional?

     

  • 0
    TI__Guru 62920 points

    Hi David,

    It sounds to me like your understanding is correct.  There are a couple ways you can think about the input jitter tolerance.  You can say that the total jitter from the transmitter and the media needs to be less than the jitter tolerance, or you can say that the total jitter of the transmitter, media, and receiver needs to be less than 1 UI.  In the latter case, the amount of jitter introduced by the receiver could be calculated as 1 UI minus the receiver jitter tolerance.  (The app note is perhaps a bit confusing in this regard, as the jitter allowed and jitter added are both 0.5 UI.)

    If you want to determine the media jitter requirements, you need to see how much margin you have left after accounting for the transmitter and receiver jitter.  For the TLK3101, the transmitter jitter is 0.2 UI and the receiver jitter is 0.4 UI.  This leaves 0.4 UI of jitter allowable in the media if you want to keep the total jitter below 1 UI.  (Said another way, the receiver can tolerate 0.6 UI and the transmitter adds 0.2 UI, leaving 0.4 UI for the media.)

    I hope this clears things up.

    Best regards,
    Max Robertson
    Communications Interface Applications
    m-robertson@ti.com

  • 0 in reply to Miguel Robertson
    Expert 1860 points

    Great, thank you so much.

  • 0 in reply to Miguel Robertson
    Expert 1860 points

    One more question about the Jitter tolerance of TLK3101.

    On datasheet the Jitter tolerance is: min 0.6UI.

    Does this mean Jitter tolerance of TLK3101 could be bigger than 0.6UI such as 0.9UI?

    Please help me to explain

    Thanks.

     

  • 0 in reply to David Zhang16358
    TI__Guru 62920 points

    Yes, although I'm not sure whether or not it could go as high as 0.9 UI.  The minimum spec means that under the range of operating conditions defined by the datasheet, the receiver jitter tolerance should not be less than 0.6 UI.

    Max

  • 0 in reply to Miguel Robertson
    Expert 1860 points

     "the receiver jitter tolerance should not be less than 0.6 UI."

    This is really a weird requirement.

    Based on my understanding, I would like jitter tolerance and receiver generated jitter as small as possible since I want get wider eye open.

    If the sum of all related jitter less than 1UI, it should be much better.

     

  • 0 in reply to David Zhang16358
    TI__Guru 62920 points

    Receiver jitter tolerance is not the amount of jitter that the receiver generates.  Rather, it is the amount of jitter that the receiver can handle on its input.  A receiver jitter tolerance of 0.6 UI means that the receiver can accurately track its input signal as long as the signal's jitter is less than or equal to 0.6 UI.  As the input jitter increases above this spec, the BER will increase as well.

    You are correct that you want receiver-generated jitter to be low.  The less jitter in the receiver, the more you can have elsewhere in the circuit.  The amount of jitter that is OK to have elsewhere in the circuit is the receiver jitter tolerance.  Therefore, you want the jitter tolerance to be high, not low.  Does this make sense?

    Max

  • 0 in reply to Miguel Robertson
    Expert 1860 points

    I totally understood.

    Thank you so much.