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LMK04806BEVAL: LMK04806BEVAL

Markus Reinhardt
Prodigy 20 points
Part Number: LMK04806BEVAL

Tool/software:

Hi all, Hi Dean Banerjee,

I have a design done with the device LMK04806B with PLLatinum Sim.



I have calculated the PLL phase noise according to the given FOM and PN10kHz metrics. The result looks very reasonable to me.
At 100 Hz frequency offset I get -96 dBc/Hz and the floor is at -109 dBc/Hz.
 
But the result is different to the shown PLL phase noise PSD in PLLatinum for higher frequency offsets,
which shows that the closed loop filter for the PLL seems to be included (what I agree to).

What I do not understand is the low frequency case:
The closed loop transfer function of the PLL should be around + 47 dB at 100 Hz, i.e. the PLL PSD should be increase at 100 Hz by this value and give -97 dBc/Hz + 47dB = -50 dBc/Hz
But it seems the increase is none (0 dB). The overall PLL phase noise is still around -94 dBc/Hz at 100 Hz frequency offset:



So I am missing something here, but what ??

Thanks a lot

Regards,


Markus

  • 0
    TI__Prodigy 200 points

    Hi Markus,

    Apologies for the delay. Dean has been out of office, and will be back in office next week.

    Best Regards,

    Andrew Lin

  • 0 in reply to Andrew Lin
    Prodigy 20 points

    Andrew,
    Thank you for the information.
    When I read my question again, I find it myself a bit unclear now.
    I rephrase again:
    My main question is, how the phase noise calculated by FOM and PN10kHz is then filtered by a PLL transfer function to calculate the phase noise at the output.
    Specifically if I look to the 100 Hz frequency offset point the closed loop PLL transfer function has a value of +47 dB, the phase noise calculated with FOM and PN10kHz has -96 dBc at that point, and if I would add them up, the phase noise value at the output would be -49dBc, but in the PLLatinum I see a value of around -96dBc, so 0 dB gain by the transfer function. So it seems that a different transfer function is used together with the phase noise calculated by FOM and PN10kHz. Where could I find the calculation of the used transfer function?

    Regards


    Markus

  • +1
    Prodigy 20 points

    Hi all,


    I believe I have now better understood my issue.
    As far as I understand now, the model of the PLL phase noise (with parameters FOM, PN10kHz and the PFD frequency is mainly related to the PFD PSD), so the closed loop transfer function of the PFD noise is to be applied (which is different to the overall transfer function. The transfer function of the PFD goes from the output of the PFD to the output of the VCO). The transfer function of the complete PLL goes from input of the PFD to the output of the VCO.
    The closed loop transfer function of the PFD phase noise converts to a low value of 1dB in my analysed case for the frequency offset going to zero. Then I understand the complete PSD curve of the PLL (~PFD) phase noise in PLLatinum.

    Reagards,

    Markus