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LMX2581: LMX2581

Meng Liu
Intellectual 1765 points
Part Number: LMX2581
Other Parts Discussed in Thread: PLLATINUMSIM-SW

Hi

when I read LMX2581 datasheet, I am confuse about Dithering. My confusion lies in why need equivalent Denominator > 200 and is divisible by 2 or 3 can enable dithering? it's very importance question is about why is 2 or 3 ?

  • 0
    TI__Guru* 81690 points

    Hi Meng,

    Most of the team members are having thanksgiving holidays, response will take a bit longer. 

  • 0 in reply to Noel Fung
    TI__Mastermind 27581 points

    Meng

    Dithering is a tool that can improve some spurs, but not others and it does increase the in-band phase noise.

    So knowing when the right time to use it is important.

    Sub-fractional spurs are those that occur at a fraction of where a fractional spur would be.  For instance, if I have a fraction of 1/100 with 2nd order modulator with 100 MHz phase detector, my primary fractional spur is at 1 MHz, but I also get a sub-fractional spur at 500 kHz.  On the other hand, if I had a fractional denominator of 125, I still have a primary fractionlal spur at 100 MHz/125 = 0.8 MHz, but there are no sub-frational spurs.  So for this example, dithering might help with the fraction of 1/100, but for the fraction of 1/125, it would not improve spurs and would add noise.

    So what the datasheet is advising is that if the fractional denominator is NOT divisible by 2 or 3, there are no sub-fractional spurs, so dithering only adds noise and does not improve spurs.

    For other cases, it is a trade-off between spurs and noise.

    Our PLLatinum Sim tool at ti.com/tool/PLLATINUMSIM-SW is a free tool we have that models fractional spurs and the impact of dithering.

    Regards,

    Dean