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LMK04828: External VCO and spur on outputs

Mahmud Harun
Prodigy 50 points
Part Number: LMK04828

On my receiver board, I can see a spur on my LMK04828  clock outputs (the spur is coupling from a spi bus on my board) . However, if I use an external VCO i dont see the spur.I am trying to figure out what sections/ circuits on LMK04828 are turned off when I use the external VCO. In other words I am trying to figure out which pin on the LMK04828 is getting coupled with the spi bus signal and resulting in the spur. I have powered up LMK using external power supplies and didn't see a difference.So most likely power pins are not the issue. I also used a signal generator to provide the  OSCIN and no improvement. So, PLL1 might not be a issue either. Any sugestions?

  • 0
    TI__Genius 9920 points

    Hi Mahmud,

    Notice SPI circuits are close to Clock group 1, Group 2 and VCO. 

    In datasheet, "If the SCK or SDIO lines are toggled while the VCO is in lock, as is

    sometimes the case when these lines are shared with other parts, the phase noise may be degraded during this
    programming."

    So we suggest to design a dedicated SPI for LMK04828, do not share SPI lines with other devices.

    After LMK04828 configuration is done, in normal lock status, it is no necessary to read or write registers.

    Regards,

    Shawn

  • 0 in reply to Shawn Han
    Prodigy 50 points

    Thanks for the response Shawn. I guess I need to add a few more details about the issue we are having. The SPI bus for the LMK04828 isnt being shared with any other device. The clock signal on a separate  SPI device is somehow coupling in on LMK and we are trying to find the victim. We see the spurs on the outputs and also on the CPOUT22 when using the internal VCO.; however, if the CP2 is tristated CPOUT2 line looks clean. Also, as mentioned before the outputs and cpout2 look clean when external VCO is used on the Fin pins. SO, which lines are inactive when the external VCO is applied? Possibly, one of those lines is the victim and bringing in the spur into the LMK.

    Regards

    Mahmud 

  • 0 in reply to Mahmud Harun
    TI__Genius 9920 points
    If the spur frequnecy is SPI clock, there are internal coupling path from SPI to VCO.