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LMX2491: Loop Filter on LMX2491 and PLLatinum Sim

Part Number: LMX2491

Hi ,

i'm using the tool , TI PLLatinum Sim, to help me to use the LMX2491.

1.According to the LMX2491 Datasheet, PLL_N = fVOC/FPD. So, we can get that FVCO = PLL_N*Fpd = 50MHz*96= 4800. But, isn't our target of FVCO 1510MHz?

2. No matter how i tried,  the value of the components are all different with the value in data sheet.  Any idea what I've done wrong here?

Thanks.

Best Regards,

Olivia

  • Hi Olivia,

    I think there is a typo error in the datasheet, the N-divider value should be 30.2 for VCO=1510MHz and fpd=50MHz.
  • Hi Neo,

    i haved tryied the parameter above- FVCO, KVCO, VCOCAP, Kpd, FPD and LBW, liked the first picture, but i can't still get the same values of the components. could you give me other parameters that i need to set but whitch doesn't show on the datasheet? or, could you please give me some advice?

    Thanks.
    Best Regards,
    Olivia
  • Hi Olivia,

    Datasheet stated the loop bandwidth only. There are three more PLL characteristic parameters. They are phase margin, gamma and T3/T1 ratio. They are already shown in PLL Sim. In your design, you have chosen them "automatic". When you click the Calculate Loop Filter button, PLL Sim will determine their values based on the default strategy. If you want to get the same loop filter values that are shown in the datasheet. you may need to uncheck all the "Auto" boxes. Type 54 in phase margin; 1.4 in gamma; 4 in T3/T1 ratio; 380 in loop bandwidth.