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LMX2581: LMX2581 Setting and spurious

akihiko yokouchi
Intellectual 340 points
Part Number: LMX2581

Please give us your advice regarding the LMX2581.
The setting frequency is Fosc=120MHz, Fpd=120MHz, Fout=1570MHz.
Changing the "VCO_SEL" setting will improve the spurious. Is this an effective setting for spurious measures?
However, it does not improve the 1560MHz spurious. What could be the cause? And is there a way to improve it?
If you check the calculation result of Spurs finder, you can see 10MHz in Crosstalk spurs. But I don't know how 10MHz is calculated.

Regards,
Akihiko Yokouchi/

1538.LMX2581.zipLMX2581_FOUT.pdf

  • 0
    TI__Mastermind 18070 points

    Hi there,

    We will get back to you next Monday.

    Regards,
    Hao

  • 0 in reply to Hao Z
    TI__Guru** 107030 points

    Hi Yokouchi-san,

    VCO_SEL determines which VCO core (we have four VCO cores) is used to start VCO calibration. If the start VCO core does not suitable for the desired configuration, the next possible VCO core will be automatically selected to continue the calibration. 

    In this case, only VCO3 is possible to generate 1570MHz output. If the calibration started with VCO2, the calibration will approach 1570MHz from the left hand side (forgive me to not tell you what exactly does "the left hand side" mean as the calibration algorithm is considered TI proprietary). On the other hand, the calibration will approach 1570MHz from the right hand side if VCO_SEL = VCO4. As a result, the VCO calibration parameters, and therefore the output characteristic, will be slightly different. Since the selection of VCO_SEL did not hurt the calibration, it does not matter which VCO_SEL is set.

    The 10MHz spurs came from crosstalk between fpd and output. 120 x 13 = 1560MHz. Output = 1570MHz, so the spurs is 10MHz. You can try make charge pump smaller, see if that would reduce the spurs a bit.

  • 0 in reply to Noel Fung
    Intellectual 340 points

    Hello. Dear Noel,
    Thank you for your advice.

    I changed CP from 0.5mA to 0.1mA.
    As a result, it was confirmed that the spurious at 1560MHz was improved by about 9dB.
    By the way, is it difficult to reduce the noise level below that of the spectrum analyzer?
    Also, is there any other way besides changing the CP current?
    Is it best to change the CP current and then secure the phase margin and phase noise (change the loop filter constant)?

    Regards,
    Akihiko Yokouchi/

  • 0 in reply to akihiko yokouchi
    TI__Guru** 107030 points

    Hi Yokouchi-san,

    You may also try using a smaller fpd, for example, 60MHz, the 1560MHz spurs should be smaller.

    Since changing fpd or charge pump current will change your existing loop bandwidth/phase margin, you may need to redesign the loop filter using the updated fpd or charge pump current.