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ADS42JB69EVM: THe phase noise of DCLK4 is very pool

xiao weihua
Intellectual 500 points
Part Number: ADS42JB69EVM
Other Parts Discussed in Thread: LMK04828, ADS42JB69, DAC38J84

hello, Jim

      I have test this board today, after power up, the dclk4 output a frequency signal , but the phase noise is very poor(about -75dBc/Hz@1KHz), after connected the usb and then set the dclk4 via GUI, the frequency can be changed but the phase noise is still very pool, I think the jitter of dclk4 is far exceed the technical specifications in the datasheet of LMK04828， what is the reason?  

  • 0
    TI__Mastermind 44680 points

    Hi weihua

    The clock group that contains DCLKOUT4 is powered down by default. The tone you are measuring there must be crosstalk from another signal, that is why the apparent noise is so high.

    To get a good signal on this output, please first power up the EVM, and configure one of the provided setting files.

    Then make the following changes on the LMK04828 Clock Outputs subtab in the CLKout 4 and 5 section:

    • Change the DCLK Type to LVPECL 2000 mV
    • Check the Output Drive Level box
    • Check the Input Drive Level box
    • Check the SDCLKout_PD box
    • Check the DCLKout_HSg_PD box
    • Check the DCLKout_ADLYg_PD box
    • Check the DCLKout_ADLY_PD box
    • Click in the check box to the right of CLKout 4 and 5 Group Powerdown to uncheck this box

    The GUI should look like this after you have made the changes.

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Intellectual 500 points

    That is useful!

    But there is something wrong under Win10 system, the GUI can not work properly, I have test two EVM board, the GUI for DAC38J84 is OK, but the GUI for ADS42JB69 is wrong. must I change to Win7 System?

  • 0 in reply to xiao weihua
    TI__Mastermind 44680 points

    Can you describe what is happening with the ADS42JB69 GUI in Win10?

    Do you get an error message during installation or use?

    Please gather some snapshots of what is happening and attached them in a Word or similar document so we can try to understand the problem and help resolve it.

    If you click the link at the bottom right of the response window it will give more options including icons to attach images or files.

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Intellectual 500 points
    I have not load the config, so I think the PLL is not locked.
    Today I loaded the config file , and the phase noise is about -123dBC/Hz@1KHz，this result is just OK, thank you jim!!!
  • 0 in reply to xiao weihua
    TI__Mastermind 44680 points
    Hi weihau
    I think the LMK04828 should be able to achieve slightly better phase noise performance with optimal settings. Figure 6 and 7 of the LMK04828 datasheet show performance around -126 to -127 dBc/Hz at 1kHz offset.
    If you need further improvements I recommend posting a new question specifically about the LMK04828 so the experts for that device can help.
    Best regards,
    Jim B