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LMH1297: Return loss

user5378759
Prodigy 90 points
Part Number: LMH1297

Hi,
May I ask you about return loss?

According to the below thread in E2E support forums, we should enable 10-MHz clock on 75-ohm outputs in measuring return loss.

-"LMH1297: Register setting"e2e.ti.com/.../746812

I set up device registers to transmit 10-MHz clock on SDI_OUT in EQ mode and SDI_IO in CD mode, and then I measured these return loss. But these Return Loss did not meet SMPTE standard. However these return loss met the regulation if I set up device registers to transmit 4K video signal.

I attach the images of return loss. Is there any concerns if return loss measured in outputting 10-MHz clock does not meet SMPTE standard?


Best regards

  • 0
    TI__Guru* 93670 points

    Greetings,

    1). Please note SDI_OUT is always in CD or Cable Driver mode and cannot be configured for EQ mode.

    2). 10MHz output clock can be used in CD mode only. The purpose of 10MHz clock is to keep alive the 4.7uF AC coupling capacitor.  Some network analyzers send out pulses at hiogher rate and thus we may not need to use 10MHz output clock even in CD mode.

    3). In EQ mode, you should just power up the device, put device in EQ mode through pin or register, and then measure the return loss. 

    4). Please refer to the figure 8 of the data sheet showing expected return loss in different modes. This is using LMH1297 evaluation board.

    Regards,, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 90 points

    Hi, Nasser

    Thank you for your reply.
    I understood that I do not need to use 10MHz output clock in measuring return loss of SDI_OUT(Loop-Through in EQ mode).

    I have an additional question.
    The both return loss of SDI_OUT(Loop-Through in EQ mode) and SDI_IO(in CD mode) I measured without output signal are similar to the figure 8 of the data sheet and meet SMPTE standards. May I take it that DUT does not have any concerns about the return loss?


    Best regards

  • 0 in reply to user5378759
    TI__Guru* 93670 points

    Greetings,

    Your understanding is correct. LMH1297 has return loss margin and it passes SMPTE return loss requirement without output signal.

    Regards,, Nasser