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LMX2572: LMX2572 Integer boundary spurious problem

PENG YUAN
Prodigy 90 points
Part Number: LMX2572


Hi

  • I have a question about integer boundary stray LMX2572RHAT. 

  • My FPD is 100MHz, FPD*33 = 3.3G, and the locked frequency shown in the figure is 3300.002mhz.
  • I feel the integer boundary stray is a little high, I don't know if there is any optimization space? 

  • 0
    TI__Genius 16845 points

    Thanks for posting on E2E. I have assigned your post to the responsible engineer. He will respond to you soon.

    Kind regards,
    Lane

  • 0 in reply to Lane Boyd
    TI__Guru** 112000 points

    Hi Peng,

    Given, fpd = 100MHz, VCO = 3.3GHz, the IBS should be at 100MHz offset. 

    The spurs you saw is 2kHz from the carrier, this is most likely due to Vcc noise or from the reference clock that you used.

  • 0 in reply to Noel Fung
    Prodigy 90 points

    Hi Fung

    I have carefully confirmed that this spurs  is not from the VCC. 

    I set the VCO frequency at 3300.002 MHz, the spurs  appears at 3.3GHz, fpd = 100MHz, but I set the VCO frequency to 3301MHz, spurs  also appeared at 3.3GHz, this is just a typical example, if I set the VCO frequency point 4001MHz, the spurs  appears at 4GHz, the spurs is always in the integer multiples of fpd, so I guess is integer boundary spurs. 

  • 0 in reply to PENG YUAN
    TI__Guru** 112000 points

    Hi Peng,

    Right, in the above examples, the IBS will be 2kHz and 1MHz. 

    Since 2kHz is in-band, it is very difficult to resolve this problem. Reducing charge pump current may help a bit.

    For 1MHz spurs, again reducing charge pump current may help. You can also use 4th order loop filter to increase filtering at 1MHz offset. 

  • 0 in reply to Noel Fung
    Prodigy 90 points

    Hi Fung

    Yes, I have reduced the Icp and there has been a slight improvement, but some frequency points are still significant. 

    Thank you very much for your support!