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LMX2615-SP: PLL Output Noise Issue

Deepak Attri
Prodigy 50 points

Part Number: LMX2615-SP
Other Parts Discussed in Thread: LMX2615EVM-CVAL, USB2ANY

I am using LMX2615 PLL synthesizer IC. I have developed a circuit/PCB around LMX2615 following the your guidelines, replicating LMX2615EVM-CVAL board. While testing, I am able to generate clean 7GHz output. But at other output frequencies, I am observing too much noise(band of frequencies). Screen shots attached. Keep every thing same, if I just replace by board with LMX2615EVM-CVAL board, I am observing clean output. Why is it so? What should I do to have clean output? 

7GHz.PNG6.84GHz.PNG

  • 0
    TI__Guru** 119790 points

    Hi There,

    I assume, after Vcc power up, you have programmed all register to get it locked to 7GHz.

    After that, when you change the frequency, you should first program frequency related registers (e.g. PLL_N, PLL_NUM), and then program R0 (with FCAL_EN = 1) to calibrate the VCO. Without VCO calibration, the frequency may not change. Looks to me your plot is due to no VCO calibration.

  • 0
    Prodigy 50 points

    Hi Noel, 

        Thanks for your reply. We tried your suggestion, but our issue is still persisting.

    On further testing, it seems that PLL is not getting locked. Not even at 7GHz!!! But somehow, whenever we set frequency to 7GHz, we are getting a clean output.

    After setting the frequency and trying to lock the PLL:

    In our Board: MUXout_TP remains at 0V and VTune_TP change from ~1V to ~0.5V.

    whereas in EVM_CVAL (after following exactly same steps): MUXout_TP changes from 0V to 3V and VTune_TP changes from ~2.6V to ~1.3V.

    Does this help in understanding the problem? How to resolve?

  • 0 in reply to Noel Fung
    Prodigy 50 points

    Hi Noel, 

        Thanks for your reply. We tried your suggestion, but our issue is still persisting.

    On further testing, it seems that PLL is not getting locked. Not even at 7GHz!!! But somehow, whenever we set frequency to 7GHz, we are getting a clean output.

    After setting the frequency and trying to lock the PLL:

    In our Board: MUXout_TP remains at 0V and VTune_TP change from ~1V to ~0.5V.

    whereas in EVM_CVAL (after following exactly same steps): MUXout_TP changes from 0V to 3V and VTune_TP changes from ~2.6V to ~1.3V.

    Does this help in understanding the problem? How to resolve?

  • 0 in reply to Deepak Attri
    TI__Guru** 119790 points

    Hi There,

    Were you programming your board and the eval board with the same software? 

    Is there a provision on your board that you can use TICS Pro and USB2ANY to program your board?