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LMX2594: Does fractional spur affects to phase sync?

user6249907
Intellectual 295 points
Part Number: LMX2594
Other Parts Discussed in Thread: LMX2592,

Can LMX2592 guarantee multi-chip phase synchronization evenif the situation fractional spurious level is higher than VCO noise?.

I assume the LMX2592 reference input signal and MASH_SEED value are same for all devices.

I'm afraid dithering (usually realized by adding tiny noise on ΔΣ modulator input) causes uncorrelated noise.

Does strong dithering is not incremented in LMX2594?

I guess so because strong line spectrum spurious appears when PHASE_SYNC is ON and Fden is small value.

  • 0
    Intellectual 295 points

    I'm sorry there is a typo correction.

    Can LMX2592 guarantee multi-chip phase synchronization evenif the situation fractional spurious level is higher than VCO noise?.

    I assume the ""LMX2594"" reference input signal and MASH_SEED value are same for all devices.

    I'm afraid dithering (usually realized by adding tiny noise on ΔΣ modulator input) causes uncorrelated noise.

    Does strong dithering is not incremented in LMX2594?

    I guess so because strong line spectrum spurious appears when PHASE_SYNC is ON and Fden is small value.

  • 0 in reply to user6249907
    TI__Guru* 81400 points

    Hi there,

    As long as it is not a Cat. 4 sync, the sync mechanism will ensure phase synchronization is possible. 

    Although we did not characterize the performance in Cat. 3 sync, especially at high output frequency, we believe that the accuracy of phase synchronization will degrade if the synthesizer output jitter is bad. 

    Jitter will increase either with bad phase noise or with spurs. 

    LMX2594 does not have the dither function. However, if you set DEN to a weird number such as 12345678, the ultimate effect is similar to dithering. Please note dithering can reduce spurs but will increase phase noise. You may end up with the same situation with or without dithering.

  • 0 in reply to Noel Fung
    Intellectual 295 points
    Thank you very much for quick response.
    In the previous question, I'm afraid that DITHER and large DEN might be different in the following points.
      Dither => MASH register values might differ for each chip
      large DEN => MASH register values match on all chips
    Does your answer indicate, phase noise and spurs degrade phase synchronization accuracy,
    regardless of the cause(CP, VCO, MASH, Loop Filter etc.)? .
    Is my understanding correct?
  • 0 in reply to user6249907
    TI__Guru* 81400 points

    Hi there,

    Yes, I believe so, phase noise and jitter may affect phase synchronization accuracy. It does not matter who created spurs and bad phase noise.