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LMX2581EVM: LMX2581EVM, The Output frequency is sometimes different from the setting.

Part Number: LMX2581EVM
Other Parts Discussed in Thread: CODELOADER, LMX2581

Dear Specialists,

My customer is confirming LMX2581EVM.

The output frequency is sometimes different from setting condition.

He sets 144MHz, but sometimes 140MHz.

Could you please advise what is the problem, device failure, setting,  programing sequence, or something ? 

(1)Please see attached file. (LMX2581EVM_codeloader_setting)

(2)Power On Sequence complies with datasheet  of 8.5.2 Recommended Initial Power on Programming Sequence.

-EVM Power ON

-12MHz refefence input

-send setting data from codeloader

I appreciate your great help.

Best regards,



  • Shinichi,
    I do not see any issues with the attached settings, but I suspect that something is going wrong with the VCO calibration. The LMX2581 divides the entire frequency range into 4 VCO cores (VCO1, VCO2, VCO3, and VCO4) and each of these 4 cores is further divided into 256 bands. If the VCO calibration chooses the wrong band, it will be off.

    For instance, if you program the device before the power is fully applied and the pins are at the proper bias levels, ,this could cause a calibration issue. Also, this could happen if the input reference is not fully running at the tine that the R0 register is programmed.

  • Dear Dean

    Thank you for your reply.

    I took a New Year Holidays and reply lately.

    I'll send your suggestion and make him confirm.

    I appreciate your great help.

    Best regards,


  • Dear Dean

    Thank you for your support.

    I confirmed with the customer.

    Test Procedure is listed below

    1. Power supply to the EVM

    2. Reference 12MHz supply.

    3. Program from codeloader

    Each process is performed manually, it has enough time to rise up properly.

    Also,  I attached codeloader setting capture.

    Is there any problem about the setting?    

    I appreciate your great help.

    Best regards,



  • Hi Shinichi,

    I also don't see any problem with the setting. Is it very easy to duplicate the problem? Is the frequency off by exactly 4MHz? What kind of 12MHz clock source are they using?
  • Dear Noel

    Thank you for your reply.

    I confirmed the customer.

    The problem is suddenly happened, not easily.

    The frequency is measured by oscilloscope, not precisely.

    12MHz reference is black box, the customer doesn't know in detail.

    He only know phase noise spec, -140dBc@10kHz, -145dBc@100kHz and -145dBc@1MHz, he has measured himself.

    I think it should be confirmed by another reference oscillator and compared.

    I appreciate your great help.

    Best regards,
  • Hi Shinchi,
    Right, try using a different reference clock, see if the problem will be gone away.