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LMX2595: Fractional spurs beyond where anticipated

John Haraguchi
Prodigy 250 points
Part Number: LMX2595
Other Parts Discussed in Thread: PLLATINUMSIM-SW

I am evaluating spurs on the LMX2595 and finding them at unexpected offsets.

The reference input is 100 MHz and the reference doubler is enabled for a 200 MHz PFD.

I have the fractional denominator set to 20 for a 10 MHz step size.

The MASH order is set to 3.

I expected spurs only near phase detector multiples, but there are spurs at nearly every tune frequency with offsets that have no correlation to difference between the tune frequency and a phase detector multiple.

I’m seeing spurs at 5, 10, 15, and 20 MHz offsets (not always all of these at once) as I tune across the band.

I plan to operate in the full-assist mode, but I did also check with the VCO auto-calibration enabled and found this made some spurs better. For example, if I use full-assist mode and tune right on a PFD multiple (i.e. 13000 MHz), there are spurs at 5 and 10 MHz offsets. If I enable the VCO auto-calibration and tune to a PFD multiple the spurs go away, but there are others as I tune away from a PFD multiple.

Are there some register settings that could be causing these spurs?

Thanks,
John

  • 0
    TI__Mastermind 28001 points

    John,

    With a phase detector frequency of 200 MHz and a fractional denominator of 20 you can get spurs at 200MHz/20 = 10 MHz and also sub-fractional spurs at 200 MHz/20/2 = 5 MHz.   You will find that if you switch to the first order modulator, the spurs at 10 MHz will go up, but the sub-fractional spurs at 5 MHz will go away.  If you switch to a 4th order modulator, you will see sub-fractional spurs at 2.5 MHz offset. 

    Fractional spurs are not a trivial subject and are modeled by our free online PLLatinum Sim tool (ti.com/tool/PLLATINUMSIM-SW).  Also fractional spurs are discussed in depth on my free PLL online manual at (ti.com/tool/pll_book)

    Regards,

    Dean