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LMX2820: Whether the NUM value can effect on output

yates.chen
Prodigy 50 points
Part Number: LMX2820

Hi Team：

When I configure the LMX2820 encountered the following problems：

1，Fosc=100M，the output is 200M；the den=1,000,000，and resolution is 100Hz；

2，when the output=200M+100Hz，the configure as follow：

it can lock！and the output is right！

3，when the output=200Mhz-100Hz：

It can‘t output，read the reg74，rb_LD=0x2，but the LD pin is low level；

4，Does it have anything to the fraction NUM and INST_CAL_PLL_NUM value?

Are there any other registers need to adjust?

5，In application, set it up via the MCU SPI.

Look forward to your help! Tks!

  • +1
    TI__Guru** 108130 points

    Hi Yates,

    I don't have problem locking to these frequencies. To simplify the debug, could you try again without Instant Calibration?

    By the way, I simplified the procedure to run Instant calibration as below.

  • 0 in reply to Noel Fung
    Prodigy 50 points

    Hi Noel:

    Thank you for your reply.I follow the above procedure,and the result doesn't change.

     whether Instant Calibration or not，when the num register value is smaller，the output is ok；

    eg.200M+10K

    when the num register value is bigger，the output is failure；the LD pin is always low.

    eg.200M-10K, The actual output is 200.17M.

    Do you have any other suggestions?

    Look forward to your help! Tks!

  • 0 in reply to Noel Fung
    Prodigy 50 points

    Hi Noel:

      Maybe I found the problem when the fvco less than 6400Mhz and the output doesn't lock!!!

      It has nothing to the value of num.

      Why the fvco can affect the output?How can I to deal with this problem?

      Hardware or software should to be focused?

      I have no idea. I hope you can help me.

      Tks!

  • 0 in reply to yates.chen
    TI__Guru** 108130 points

    Hi Yates,

    I don't know what happen on your side, but here my test result. Make sure you click the "Calibrate VCO" button every time you have change the frequency.

  • 0 in reply to Noel Fung
    Prodigy 50 points

    Hi Noel：

    Thank you for your help.

    The problem has been found.

    Before the calibration, the INSTCAL_EN bit must be clear to 0 every time.

    I was just reset the FCAL_EN bit.

    Thanks again.Wish you all the best.