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LMX2595: How to reduce IBS and IBS/2 Spurs

Xiaolan Ge
Prodigy 10 points
Part Number: LMX2595

 We tested LMX2595 EVK recently  and besides Fpd,x2,X3, IBS spurs,  we observed spurs located IBS/2. We also noted that the IBS spur level varies when  using different value for PFD_DLY_SEL ( R37). 

we are looking for more information about:

 

  1. What is root cause of IBS/2 spurs that we observed ?

 2. What is  the function of  the “PFD_DLY_SEL” bits in R37 ?  (Are we changing phase or amplitude or ?)

 3. Do the setting of “PFD_DLY_SEL” bits give stable performance over temperature?

 4. Does  improvement of the IBS spur level by changing the “PFD_DLY_SEL” impact other performance parameters?

  • 0
    TI__Guru* 99700 points

    Hi Xiaolan,

    If your output is not direct VCO output, you may get IBS/CHDIV. 

    PFD_DLY_SEL is a delay added in phase detector in order to reduce fractional spurs. The amount of delay cannot be too big otherwise phase noise will get worst dramatically. Follow datasheet suggestion to set the PFD_DLY_SEL, this suggestion has taken temperature into account.

    IBS will not get improved with PFD_DLY_SEL. Smaller charge pump current usually can reduce IBS. Alternatively, use a different fpd to avoid IBS.