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LMX2820: PLLatinum software, PFD frequency not changing when input reference frequency changed

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Replies: 1

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Part Number: LMX2820


I simulated LMX2820 with 100MHz Input Reference frequency, PFD 200MHz, Output 12000MHz. I changed Input reference frequency to 99.99991MHz, but PFD frequency was still 200MHz multiplier changed to 2.00000180000162. Please let me know whether it will work the same in real time or it is software issue.

  • Hello Pathamuthu,

    The multiplication or division factor is not fixed in many devices (including LMX2820). The input multiplication or division factor should highlight red when the multiplication is invalid i.e. not realizable within the device. But it appears that the box is not properly highlighting when the relative change in Fosc is within 0.1% of the input frequency. Similarly, it seems that changing the phase detector frequency does not indicate a warning if the Fosc frequency is unrealizable. I'll add this to the list of improvements for the next update.

    In practice, each device only has a certain set of physically realizable R-path multiply/divide coefficients, so a value like 2.00000180000162 is not realizable for the input multiplier of the LMX2820.

    The reason we allow non-realizable values for the input multiplication or division factor is for flexibility in reference source modeling. For example, if I have the phase noise trace of a 100MHz oscillator, and it scales similarly to a 122.88MHz oscillator from the same vendor which I would like to use but for which I do not have the phase noise trace, I could simulate using the 122.88MHz oscillator in PLLatinum Sim by loading the 100MHz Fosc data and setting Fpd to some factor of 122.88MHz, resulting in a non-realizable multiplication factor but an accurate prediction of the scaling of the oscillator noise.


    Derek Payne

    Texas Instruments