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DS90C385A: Radiated Emissions Issues from Harmonics of 38Mhz TXCLKIN

Rabbit
Prodigy 50 points
Part Number: DS90C385A

Hi, I am currently using the DS90C385A LVDS driver with a BeagleBoneBlack (BBB) and 7" LCD.

I am experiencing radiated emissions issues from the harmonics of the 38Mhz clock which originates at the BBB and connects to pin31 of DS90C385A.

I have ferrite beads and decoupling on each Vcc - Gnd inout of the IC but no filtering on the data in or data out from the LVDS IC.

Could you provide an application note or solution describing filtering to be implemented on the I/O data lines.

I have tried a ferrite bead on the clock at source with little effect so I assume the harmonics are propagating through system some other way.

A solution to implement at the BBB and/or LVDS IC would be most helpful.

Many thanks.

  • 0
    TI__Intellectual 2915 points
    Hello,

    There are different ways to reduce the radiated emissions. What kind of cables do you use on the system? Are they shielded STP?

    Have you looked into snlu032a? Please see the schematic on page 16.

    Dennis
  • 0 in reply to Dennis Wu
    Prodigy 50 points
    Thank you for the reply Dennis. Since that time I have done some further investigation which shows that the radiated emissions issues are present at the BeagleBoneBlack (AM3358BZCZ100). Researching this forum indicates many people have had similar issues but there seems to be a lack of a definitive solution to this. Spread Spectrum Clocking (SSC) has been mentioned as a potential solution but this is noted as not supported for the DISP PLL Core which is where the LCD_PCLK in question ultimately originates. Any help with this particular point will much appreciated.
  • 0 in reply to Rabbit
    TI__Intellectual 2915 points
    Hello,

    Can you send the schematic and block diagram to us? What is your data rate?

    For the EMI reduction, below are a few tips:
    • Ground the TFT's bezel
    • Utilize flat cable Ferrite core (used for FPC/FFC cable)
    • Utilize EMI shielding foils and film

    I don't have too much experience on BeagleBoneBlack (AM3358BZCZ100).

    Regards,
    Dennis
  • 0 in reply to Dennis Wu
    Prodigy 50 points

    Hi Dennis,

    The clock in question is 38.4MHz, this is the LCD_PCLK originating from the BeagleBoneBlack (BBB).

    If I disconnect the display, the emissions issues are still present so i think any shielding or similar solutions applied at the display end would have limited effect on the problem.

    I feel it needs to be tackled at, or as close as possible to, source.  I.e. the AM3358BZCZ100 or on the BBB.

    The BBB Schematics are attached; the LCD_PCLK is shown on sheet 4 of 11.

    Many thanks for your assistance.

    BBB Schematics.pdf

  • 0 in reply to Rabbit
    TI__Intellectual 2915 points
    Hi,

    After reading your threads, the radiation emission is coming from the BBB. I agreed ground the LCD may not help because it's not the source of radiation emission. It's hard to rule out and provide you an advice on BBB because we don't have experience on this.

    Do you have a block diagram and schematic for DS90C385A and BBB connect together?

    What kind of cables and connectors do you use? Is it shielded?

    Dennis