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LMK03318: frequency margin

Penn Zhang
Intellectual 1180 points

Part Number: LMK03318

Hello,

Could you kindly help on the questions below, thanks?

1. Use REF1 for 25MHz reference, but it is 24.9999MHz, do you think this will cause the output frequency is a bit lower than set?

2. How many methods to fine-tune these output frequency?  Could change the fraction-divider works? any other method?

3. If I change the fraciton-divider register while the PLL working, will the setting active immediately? or I need power on reset?

Thanks.

  • 0
    TI__Mastermind 18070 points

    Hi Penn,

    The PLL output will follow the reference input precisely. For example, your reference input is off by -4ppm (-0.0001/25*1e6) then your output will also be off by -4ppm. So instead of, say, 100MHz output, it will become 99.9996MHz.

    There are two methods to fine tune output frequency. One is by tuning numerator of fractional divider, the other is to fine tune the load capacitance the crystal, if an XTAL is used. In general, load capacitance tuning is recommended because it introduces no degradation in phase noise. Fractional numerator tuning, on the other hand, can add additional fractional spurs. Please refer to this appnote for more details: http://www.ti.com/lit/an/snaa281a/snaa281a.pdf

    A soft reset is needed after changing the fractional divider in order to recalibrate the PLL. 

    Regards,
    Hao

  • 0 in reply to Hao Z
    TI__Intellectual 1180 points

    Thanks a lot, Hao.