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LMX2615-SP: Fractional Simplification

Bill Grove
Prodigy 120 points
Part Number: LMX2615-SP

In using the fractional divider Fnum and Fden, is it required to simplify the Fnum/Fden fraction?  For example say I want to keep Fden = 30 always, and let Fnum vary from 0 to 29.  Is it ok to set Fnum = 15 and Fden = 30, or is it required to simplify the fraction to Fnum = 1 and Fden = 2?  Are there any disadvantages to not simplifying the fraction?

thanks,

Bill

  • 0
    TI__Genius 11900 points

    Hello Bill,

    The short answer is no, there are no disadvantages in having a bigger denominator.

    The denominator comes into play when you notice spurs which are multiples of:

    phase detector frequency / (k * simplest form of denominator)

    where the simplest form of the denominator is 2 in either case 1/2 or 15/30 and k can be 2,3,4,6 or 12.

    Using a fractional value of 1/2 vs 15/30 will theoretically not change the location or magnitude of the sub-fractional spurs.

    If these spurs are undesired then a fraction such as 10000/20001 can be used, however this will increase the phase noise of the PLL and will have a very small frequency error. The LMX2615-SP does not have a dithering function and this helps us to replicate the same.

    Thanks,

    Vibhu

  • 0 in reply to Vibhu Vanjari
    Prodigy 120 points

    Hi Vibhu,

    It is good that one does not need to simplify the fraction in the divider chain as it simplifies programming especially for broad frequency coverage.

    thanks,

    Bill