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LMX2595: Two frequencies that are 10MHz apart when I programmed LMX2596 at 7.8GHz

LIU
Prodigy 220 points
Part Number: LMX2595

Hi, Ti

Now I am using LMX2595 to generate frequency at 7.8GHz in no assist mode. However, I observed two frequencies which are 10MHz apart, and I don't enable the ramping part. Below are my configurations in TICS pro. 

When I set R0[2] in readback mode, all the registers have the same values as I programmed in, except for the registers between R107-R112.

Could you please help me find where I am wrong? Thank you!

  • 0
    TI__Mastermind 33710 points

    LIU,

    It doesn't look like there's any error in TICS Pro configuration. Is it possible that your reference source is not exactly 100MHz? If you put the reference source into the spectrum analyzer, do you see exactly 100MHz?

    Regards

    Derek Payne

  • 0 in reply to Derek Payne
    Prodigy 220 points

    The reference clock is provided by a separate 100M crystal oscillator. From the spectrum, there is a correct frequency 7.79M. And I try different frequencies, there always exists two frequencies that are 10M apart.

  • 0 in reply to LIU
    TI__Guru** 108130 points

    Hi Liu,

    Could you make a smaller span so that we could see the signal better? The noise floor of your plot is too high, we may be losing some useful information. 

    In addition, was the test result given from your own board or from our EVM?

    From your R112 read back data, looks like the DACISET is not correct, usually it will not exceed 400. Did you put it in full assist mode or no assist?