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Original question:

DP83848K: Problem for TD+/TD- pin waveform

DP83848K: Problem for TD+/TD- pin waveform

Toshihiro Watanabe
Genius 10235 points
Part Number: DP83848K

Hello,

DP83848K TXD waveform is measured as attached. The high frequency noise on the signal is observed.

This high frequency noise can observe in 10Mbps case. 100Mbps case is no issue.

[Question]

1) What is the thinkable reason we can observe the noise on signal? I think we can see clean signal if EVM is used.

2) Would you share me DP83848K EVM TXD waveform of 10Mbps for the reference? We want to confirm what signal is proper signal quality. 

Best regards,

Toshihiro Watanabe

  • 0
    TI__Genius 10235 points
    Hello,
    Is it possible I can see DP83848EVM TXD waveform in case of 10Mbps and 100Mbps? I would like to compare the waveforms between the customer and EVM one.
    Best regards,
    Toshihiro Watanabe
  • 0 in reply to Toshihiro Watanabe
    TI__Mastermind 19780 points
    Hi Watanabe-san,

    Can you provide a schematic of the DP83848K? I will generate a 10M waveform from the DP83848K by tomorrow 4/24 in the US for your observation.

    If you have noise on the line driver, this is typically due to noise on supply voltages, or impedance mismatch by either a poorly controlled characteristic impedance on the PCB, or no load termination when probing the MDI.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    TI__Genius 10235 points

    Hello Rob-san,

    Thank you for your support. I will share the schematic offline but it will be the same as below post.

    Best regards,

    Toshihiro Watanabe

  • 0 in reply to Toshihiro Watanabe
    TI__Mastermind 19780 points

    Hi Toshihiro-san,

    Attached are scope shots of the DP83848's output waveforms, as measured on our bench from the EVM.

    Note that the same signal appears to have noise on it, when probed directly between the DP83848 and the transformer.  When probing across the 100 ohm load, there is no noise.  This is to be expected, and your probe point should be across a 100 ohm load.  This goes for both 100M and 10M modes.

     measured between DP83848 and transformer

     measured across 100 ohm load.

    Best Regards,

  • 0 in reply to Rob Rodrigues
    TI__Genius 10235 points
    Hello Rob-san,
    Thank you for the waveform. Let me confirm something. We measured at between DP83848 and transformar. In the same condition, we can observe same noise on the EVM. Is my understanding correct?
    I think the configuration is DP83848 -> transfomer -> 100ohm load. We measured at 100ohm load then noise was not observed. So we need to check if the noise is observed at the cable side of transformer. Is my understanding correct?
    Best regards,
    Toshihiro Watanabe
  • 0 in reply to Toshihiro Watanabe
    TI__Mastermind 19780 points
    Hello Toshihiro-san,

    Yes, your understanding is correct.

    If probing here, you will see noise: DP83848K <-Probe-> transformer <---> RJ-45 <---> 100 ohm load

    If probing here, you will not see noise: DP83848K <---> transformer <---> RJ-45 <--->[Probe] 100 ohm load

    This is how my scope captures were made.

    Best Regards,