I'm using the HMC832 in the conditions fixed in the datasheet p34 figure 52 (loop filter design).

We choose the loop filter type 1 in the table12. The Fxtal is 40Mhz.

Depending of the output frequency desired, the HMC832 declares an unlock condition or not.

I noticed that the PLL was locked when the fvco (=k*fout) was high, close to the 3000Mhz.

For example :

  • fout = 500Mhz, k=6, Nint=75 et Nfrac=0 (Registre3=75, Registre4=0) -> the PLL is locked
  • fout = 500,1Mhz, k=4, Nint=50 et Nfrac=0.01 (Registre3=50, Registre4=167772) -> the PLL is unlocked
  • until fout = 629.4Mhz, k=4, Nint=62 et Nfrac=0.93999996 (Registre3=62, Registre4=15770584) -> the PLL is unlocked
  • from fout = 629.5Mhz, k=4, Nint=62 et Nfrac=0.95000400 (Registre3=62, Registre4=15938422) -> the PLL is lockedto fout = 750Mhz, the PLL is locked.

I configure only the registers not used with default value:

REG1 0x3

REG7 0x214D

REG9 0x603264

REG5 0xB238

REG5 0x5D98

REG5 0

And then, each time I configure a new frequency:

REG5 XXXX with the k value

REG5 0

REG3 and REG4 with the new value

What do you think of my initialization ?



  • Hi,

    I did some tests for additional informations : I programmed a HMC832 in the conditions above.

    From 88Mhz to 250Mhz with an interval of 0.1Mhz, the PLL is always locked.

    The first time the PLL is unlocked is for 250.1Mhz.

    Thanks for your help.


  • +1
    •  Analog Employees 
    on Feb 22, 2021 4:55 PM 1 month ago


    Your initialization and update sequence looks correct. REG6 should be updated to a different value (0xF4A) than the default. Can you add that? It is better to write all registers. You can use the register file given in the GUI package as a reference.

    Can you please confirm..
     this is your own board, not the eval board?
     the PLL is really not locking rather than just the lock detect fails?

    Is it possible to check the PN profile to confirm there is no issue with the loop filter, no peaking?

    What is changing when transitioning from 250MHz to 250.1MHz? Does VCO jumps from 3000MHz to 1500MHz for the first time?


  • Hi,

    Thanks for your answer, it helps.

    I list all the registers left with default value. I updated REG6 with 0xF4A and REGA with 0x85.

    Now, the PLL is always locked.

    In the register 0x0A, I wonder why the default value of bit 9:3 is 64 instead of 8 as it is specified in the description ?


  • 0
    •  Analog Employees 
    on Feb 24, 2021 9:11 AM 1 month ago in reply to CBardou


    It is great to hear the problem is solved.

    Are you asking in a general manner or specific to register 0x0A?

    Those bits are used to optimize the autocalibration process. The default (hard-coded) value is expected to be the best choice in design stage (by simulation). But when testing the part, another value is found that gives better results, or even the default value may fail at all. In this case a new value different than default is recommended. Is that what you are asking?



  • Hi

    Yes, I was asking in a general manner.

    I understand what you mean. Thanks for these explanations.