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LMX2595EVM: LMX2595 certain output frequencies close in spurs

Teach Me
Expert 1940 points
Part Number:

Hi TI Experts,

I’m currently using TICS Pro to control the LMX2595 and am facing a problem which I hope you can help me with:

Problem:

At certain output frequencies especially when a fraction is used, close in spurs were seen with their severity depending on the order of modulator selected.

Do you have any suggestions to remove close in spurs within 10MHz offset from the carrier?

 

Example:

At 7500.2MHz VCO output

1st order modulator 

Spectrum and Phase Noise

2nd order modulator

Spectrum and Phase Noise

 

3rd order modulator

Spectrum and Phase Noise

 

 

*Remark:

-1st and 3rd Order Modulators give very bad phase noise.

-While the 2nd order modulator gives better performance, close in spurs are still present

- This issue does not occur at 7500MHz or whole number frequencies where fractions are not used

 

At 7500MHz via VCO

1st Order Modulator

 

2nd Order Modulator

 

3rd Order Modulator 

Phase noise measurement

 

Do you have any suggestions to remove close in spurs within 10MHz offset from the carrier?

Thanks,

Br,

Teach Me

  • 0
    Expert 1940 points

    Data.docx

    Insert File

    Part Number: LMX2595EVM

    Hi TI Experts,

    I’m currently using TICS Pro to control the LMX2595 and am facing a problem which I hope you can help me with:

    Problem:

    At certain output frequencies especially when a fraction is used, close in spurs were seen with their severity depending on the order of modulator selected.

    Do you have any suggestions to remove close in spurs within 10MHz offset from the carrier?

     

    Example:

    At 7500.2MHz VCO output

    1st order modulator 

    Spectrum and Phase Noise

    2nd order modulator

    Spectrum and Phase Noise

     

    3rd order modulator

    Spectrum and Phase Noise

     

     

    *Remark:

    -1st and 3rd Order Modulators give very bad phase noise.

    -While the 2nd order modulator gives better performance, close in spurs are still present

    - This issue does not occur at 7500MHz or whole number frequencies where fractions are not used

     

    At 7500MHz via VCO

    1st Order Modulator

     

    2nd Order Modulator

     

    3rd Order Modulator 

    Phase noise measurement

     

    Do you have any suggestions to remove close in spurs within 10MHz offset from the carrier?

    Thanks,

    Br,

    Teach Me

  • 0 in reply to Teach Me
    TI__Expert 5370 points
    Dear Teach me,

    All pictures you have placed in your question are not visible. We can answer your question but it would be helpful if you could upload the pictures.

    Regards, Simon.
  • 0 in reply to Teach Me
    TI__Mastermind 28001 points
    Teach me,

    The pictures would be helpful, but some other things you can do:
    1. Be sure to check out our PLLatinum Sim software at www.ti.com/.../PLLATINUMSIM-SW
    2. You can check out our online PLL book that discusses PLLs and fractional sprus in depth at www.ti.com/.../PLL_Book

    In regards to the LMX2594 EVM, this has a very wide loop bandwidth for optimal Jitter/EVM for integer channels. As the bandwidth is wider the offset of your fractional spurs, you will likely see some spurs. So some things to consider are:

    1. Lower the loop bandwidth for better spur attenuation
    2. Use the tools above for better idea of how spurs behave.
    3. If you are using 200 MHz phase detector frequency, maybe the reason for the higher spurs is the N divider is lower. This might be why 2nd order is better. Also, be sure that you set PFD_DLY_SEL correctly in TICSPRO. Boxes will turn orange to alert you; if you ignore these warnings, it could hurt spur perform,ance.
    4. If using your own reference, higher frequency and higher slew rate is better for spurs.

    Regards,
    Dean