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LMX2572: Phase noise at certain frequency varies with different output power settings

Mark
Prodigy 57 points
Part Number: LMX2572

R&S FSPN26 is used to evaluate the phase noise of LMX2572 evaluation board at 3.139GHz.

It is found that changing the power configuration word will change the phase noise. From pwr = 10 to pwr = 63, the maximum phase noise difference is 7dB.

Whether it is VCO output or divided by 4 output, divided by 8 output, its phase noise will not change with power varying, only VCO being divided by 2 will have this phenomenon.

What I wonder is whether this phenomenon is normal?

Here are the TICS pro setting and measured phase noise with different power settings.

  • +1
    TI__Guru* 88890 points

    Hi Mark,

    I see the same issue in the EVM. I don't have a good answer for this behavior, probably it is due to noise coupling when CHDIV = div/2. I have tried some hidden registers, it does not seem that there is a way to resolve this problem with register setting.