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LMX2572: How smooth is the frequency transition

Saied Benyamin
Prodigy 90 points
Part Number: LMX2572

I want to use the LMX2572 to close the loop on data recovery using a software CDR.  The output clock of the LMX2572 will be 5.625GHz +/- 100ppm. I will use software to increase/decrease the frequency until it is locked. I do realize that the loop time constant is very long, but this is fine.  My question is whether the transition of up to +/-50ppm in frequency will be smooth or will it generate high jitter in the output at the time of transition. Assuming that I do not need to recalibrate within this +/-100ppm range.

Thank you

  • +1
    TI__Guru 54730 points

    Hi Saled,

    100ppm on 5625MHz = 562.5kHz, this is a very small amount of frequency change for the VCO, a VCO calibration is not necessary. 

    How are you going to change the frequency? If you change the VCO frequency by changing the fractional numerator, then the PLL will be in fraction and spurs are expect. If you can, a better way off is put the PLL in integer mode and then change the frequency of the reference clock. With this approach, you will always in integer mode with no spurs and optimized phase noise.

  • 0 in reply to Noel Fung
    Prodigy 90 points

    Thank you for your help. Got it.