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LMX2594: LMX2594 configuration

Theodoros Kontogiorgis
Prodigy 20 points
Part Number: LMX2594
Other Parts Discussed in Thread: LMK04208

Hello,

I am quite beginner with PLL's neither have strong theory background. So i would like to know some basic stuff. We have the ZCU111 evaluation board with LMK04208 and three LMX2594. As i can understand the LMK04208 feeds  the with 122.88MHz (in the current setup) LMX2594 PLL's to produce the RFdac-adc required frequencies. I have downloaded the TICS Pro tool (which is a very helpful tool ) to produce and export the required registers for the LMX2594.
So , in case i want i.e 250MHz instead of 245.76MHz (the second is an easy frequency to produce because it needs integer PLL , please correct me if i am wrong) should i make the PLL fractional or should i programm the LMK04208 to the appropriate frequency to have again integer PLL in LMX2594? Are there any disadvantages using fractional PLL? Which is the best of the two ways to achieve the desired PLL frequency?

Thanks in advance,
Your sincerely,

Theo Kontogiorgis

  • 0
    TI__Genius 12705 points

    Hello,

    There are some trade-offs in using fractional N PLL vs. integer and what is better depends on the specific system use case. In a Frac N, there is more potential for spurious frequencies  due to the fractional divider. Using a fractional N divider also allows for a higher phase detector frequency which can reduce in band noise floor.

    You can do either way you suggest, but perhaps most straightforward would be to change the LMK2594 output to 125 MHz with fractional N divider.  If there is any impact from fractional spurs to system performance then you can consider changing the reference input to 125MHz.

    Please check our TI precision labs for new training material on PLL's which will be available by end of January.

    Regards,

  • 0 in reply to Liam Keese
    Prodigy 20 points

    Thank you very much for your quick response, really appreciated.

    Best Regards

    The Kontogiorgis,

    Noesis Technologies