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TLK105: effect of common mode filter

Shinichi Inoue
Guru 19701 points
Part Number: TLK105

Dear Specialists,

My customer is considering TLK105 and has a question.

I would be grateful if you could advise.

---

TLK105 sometimes reflect link pulse to TD+/- and hub become false link.

In this case, it is effective to add common mode filter at device side.

It is recommend by design guideline (SLVA531A).

I can't understand why common mode filter is effective.

I think since link pulse is differential, reflected link pulse is also differential.

In the meantime, common mode filter pass through differential signal.

I don't understand the mechanism.

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Genius 17990 points

    Hello Shinichi-san,

    Thank you for using the TI forum. Our product expert will get back to you by Tuesday.

  • 0 in reply to Mitch M
    Guru 19701 points

    Hi Mitch,

    Thank you for your reply.

    I am looking forward to hearing from you.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 18425 points
    Hello Shinichi-san,

    Common mode chokes provide a high impedance to incoming signals which can cause reflections. When CMCs are placed on the cable side, in certain conditions the reflections are strong enough to reach back to the host device and cause false link ups. In such cases, if the CMCs are placed on the device side then the signals coming from the cable first encounter the transformer and then the CMC. In this configuration, the reflections are not strong enough to go back to the link partner to cause a false link up. This behavior is related to non-ideal behavior of passive components.

    -Regards,
    Aniruddha
  • 0 in reply to Aniruddha Khadye
    Guru 19701 points

    Hi Aniruddha,

    Thank you for your reply.

    I'll share your suggention with the customer.

    If  they have an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 19701 points

    Hi Aniruddha,

    I sent your advise to the customer, but he couldn't solve his question.

    I'm sorry to bother again, could you please advise.

    ---

    Why CMC is effective at only device side

    though the Link pulse is differential.

    CME should be only effective for common mode signals.

    On the other hand, CMC certainly suppresses link pulses. 

    ---

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

    effect of CMC.pptx

  • 0 in reply to Shinichi Inoue
    TI__Mastermind 23675 points
    Hi Shinichi,

    Even though the signaling is differential, there is insertion losses through the transformer.
    This insertion loss helps to reduce the amplitude of the pulse, which is why the CMC should be on the IC side.

    Kind regards,
    Ross
  • 0 in reply to Ross Pimentel
    Guru 19701 points

    Hi Ross,

    Thank you for your advise.

    I think this information will help the customer better understand.

    If he has more question, I will consult to you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi