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LMX2594: Using doubler to satisfy minimum multiplier frequency requirment

Ryuji Maeda
Prodigy 10 points
Part Number: LMX2594

Tool/software:

I am creating a prototype receiver  board with the LMX2594 PLL IC. My crystal
reference oscillator frequency is little above 16MHz. For the receiver frequency plan, I need
to use ~ 80MHz comparison frequency for the PFD. I can achieve that with multiplier set to
x5. However, I noticed that the minimum input frequency requirement for the multiplier
stage is 30MHz. Can I set the reference path as shown below and satisfy the multiplier
input frequency requirement while I can achieve my target PFD frequency?

  • 0
    Prodigy 10 points

    The last sentence states "The input multiplier and OSCin doubler should not both be used at the same time"  so this won't work.

    I will use un-multiplied reference frequency and double the N-divide ratio... not good for phase noise but at least the RX will be functional.

  • 0 in reply to Ryuji Maeda
    TI__Guru** 107590 points

    HI Ryuji,

    Right, with 16MHz input, the max. fpd you can use is 32MHz. To get better PLL noise, you'd better use CMOS oscillator. 

  • +1
    Prodigy 10 points

    Hi,

    With 32MHz comparison frequency, I can still get the desired output frequency using1/2.5 division ratio... My preference is to use the chip in integer mode, but I can surely experiment with the non-integer division ratio.

    The reference oscillator is a custom super low noise oscillator. Its output is of clipped sine type, so my plan is to use a TI clock distribution IC to convert it to 50% duty cycle waveform and also differential.

    Thanks