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DS90UB954-Q1: Common Mode Choke in FPDLink PoC Mode

Carlos Rodriguez1
Intellectual 345 points
Part Number: DS90UB954-Q1

Dear Ti, 

We are wondering some issue in ALSE tests in the frequency range of 300-400MHz, we are currently using 954 together with 953 in sync mode. We have two 953 connected to the 954. Because of these failures, and because when we use ferrite clamps in the coax cable we solve this problem, I would like to ask you if could be possible to place a Common Mode choke in this lines, I think is not possible but I would like to ask you. Because we are working with PoC and Coax, not with STP/STQ.

As an information, we have Leoni Dacar 462 and 302, and we have two interconnections in the middle.

Please if you need something else, please ask me

  • 0
    TI__Genius 11340 points

    Hi Carlos,

    Using a CMC for coax application will not help because Rin- is terminated.

    Best,

    Jiashow

  • 0 in reply to Jiashow Ho
    Intellectual 345 points

    Hi Jiashow, 

    In addition to check the impedance and S parameters of our PCB and cable and perform Margin Analysis to fix the gain value, do you have other recommendations to ensure the immunity of the FPDLink?

    I have been reading:

    http://www.ti.com/lit/an/snla272/snla272.pdf

    But I would like to ask you specifically related to inmunity,  ALSE and BCI. 

    Thanks!

  • 0 in reply to Carlos Rodriguez1
    TI__Genius 11340 points

    Hi Carlos,

    I would recommend running the margin analysis and set the optimal gain setting first. Try to eliminate interconnects of you have them, or make sure the interconnect is not near the aggressor. 

    If you need to do a board re-design, definitely measure the s-parameter and TDR to see where reflection is occurring on the board.

    Best,

    Jiashow

  • 0 in reply to Jiashow Ho
    Intellectual 345 points

    Dear Jiashow, 

    Here you can find our Margin Analysis of our system:

    In my opinion and because we are using 2m cable (Around 20cm of Leoni Dacar 462, about 1.5M of Leoni Dacar 462 and 20cm of Leoni Dacar 302), It seems our Margin Analysis is a bit poor, because you have metrics with 15m cable, which have better Margin Analysis, we will need to review or completely system, Do you agree?

  • 0 in reply to Carlos Rodriguez1
    TI__Genius 11340 points

    Hi Carlos,

    The margin doesn't look too bad, especially considering there are two interconnects in the system. Having interconnects can make the margin smaller. If your system is only going to be using 2m cable then this margin should be fine. You can also consider upgrading to DACAR302 cables. They tend to be less lossy.

    Best,

    Jiashow

  • 0 in reply to Jiashow Ho
    Intellectual 345 points

    Hi Jiashow

    Thanks, I will try with Leoni 302, but I think this wont solve our problem in Immunity, we have been checking the S11 parameter in the Serializer side, and we are near 5dB in the range between 1-2GHz, so we are out from your recommended specs (954 Datasheet) correct me if I am wrong.

    This could be part of the problem. Anyway I would like to ask you, if you suffered problems before related to this test, and If there is any recommendation, for example to change the parameter of the EQ and SFILTER, of course fix the limit values of que EQ and Strobe to the parameter we obtained in the Margin Analysis....

    Thanks and regards!

  • 0 in reply to Carlos Rodriguez1
    TI__Genius 11340 points

    Hi Carlos,

    From the software perspective, you should definitely set the min/max for the EQ and strobe position. You can manually set the min/max for these parameters via registers 0xD5 and 0x41. Check the app note for more information: http://www.ti.com/lit/an/snla301/snla301.pdf

    Best,

    Jiashow