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LMX2594: Flat phase noise FOM

Des Coghlan
Prodigy 100 points
Part Number: LMX2594


 Is -238dBc/Hz the correct flat phase noise FOM? The measured @15G in Fig 6 is -104.8dBc/Hz over 20k to 400k with 100M ref , Fpd=200M (Ref doubler enabled).

20Log(15G/200M) = 37.5dB;  10log(200M) = 83 dB.

Expected measured noise floor = -238+(37+83) = -117.5dBc; this is 12.7 dB higher than measured = -104.8dB.

What is wrong with this??

  • 0
    TI__Expert 8505 points
    Des,

    Good question. The graphs we have in the datasheet use all sorts of loop filter design. Only a loop filter that is designed for very wide loop bandwidth (higher than 1MHz) will the PLL flat noise get showned. Because otherwise without a wide enough loop bandwidth some of the noise seen in the flat portion is actually some noise from the VCO.

    I think you may be using a different version of datasheet. On the latest one, look at "Figure 14. Calculation of PLL Noise Metrics". This one you can see with a wide loop filter design. Our FOM is -236dBc/Hz. Same math you used, -236 + 37.5 + 83 = -115.5 dBc/Hz. You see on the graph that is where the red line is. And the black line is VCO contribution, so with the addition of both those lines you get the actual measured number.

    Hope this is clear!

    Regards,

    Brian Wang