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LMK03318: Combination of the nominator and denominator of fractional divider

user4779049
Expert 2345 points
Part Number: LMK03318

  Hi Team,

 

Please advise me on my question bellow

 

There are plenty of possible combination on fractional divider for the PLL in LMK03318.

For example, when 44 + 1/2 is needed to set to the fractional divider,

nominator and denominator can be either 1 and 2, or 1000 and 2000 etc.

Question:  Are there any performance difference between 1/2, 1,000/2,000 or

                10,0000/20,000.?

                Do you need reduction in order to get best performance?

Mita

  • 0
    TI__Mastermind 18070 points

    Hi Mita,

    Short answer is: I recommend making the denominator larger than 10,000.

    This impacts the spur/dithering performance and the theory and explanations are pretty complex. High level idea is that by setting large num/den values you make the spurs scatter in a wide frequency range. In general, having denominator >10,000 is our rule of thumb.

    Regards,
    Hao

  • 0 in reply to Hao Z
    TI__Mastermind 18070 points

    By the way, there's a wizard in LMK03318 Ticspro software which helps you select the right VCO frequencies by providing "scores" to different solutions.

    Regards,
    Hao

  • 0 in reply to Hao Z
    Expert 2345 points

    Hao-san,

     

    Thank you for the answer.

    Usually I relay on TICSPOR to determine the PLL parameters.

    However, this case, I need to output arbitrary frequency.

    In order to get PLL parameters, I use system micro-controller.

    Aiming fine frequency resolution, I try to use 10,000,000 as the

    denominator.

     

    Mita