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LMH1983: How to adjust PLL1 lock

Part Number: LMH1983

Hello Expert,

My customer is considering LMH1983.
We think 0x1C(Loss of Lock Threshold) and 0x2D(Lock Step Size) is the setting of judging whether PLL is locked.
In their evaluation, then they used 0x1C and 0x2D register as default setting, they sometime observed PLL is unlocked.
Would you tell me how to adjust these register's to achieve certain lock of PLL?
Their test condition is as follows.

  • VDD:3.3V
  • Hin:H signal input
  • Vin,Fin: tied to High
  • XOin: 27MHz single-ended input

I'm looking forward to hearing back from you.


Best regards,
Kazuki Kuramochi

  • Hi Kuramochi-san,

    Can you confirm only the lock detect say unlock but the PLL is indeed locked? That is, you can get the right signals from the device but the lock detector say unlock?

    Do you have the specification of the VCXO? I am particularly looking for the Vtune (or Vcontrol) pin input impedance of the VCXO.

    The last question I have is, is there a op-amp buffer between the loop filter and the Vtune pin of the VCXO?

  • Hi Noel-san,

    Sorry for my late reply.
    I contacted you by mail.


    Thank you and best regards,
    Kazuki Kuramochi