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LMX2582: VCO frequency tuning

Oscar Guevara
Intellectual 2245 points
Part Number: LMX2582

Hello,

I am trying to configure the LMX2582 to output a frequency of 2304 MHz, however I am not able to acquire the wanted frequency. Using the included equation for the VCO I have calculated for my values to as such:

Fpd: 122.88MHz

PLL_N_PRE: 2

PLL_N: 18

PLL_NUM: 3

PLL_DEN: 4

FVCO = 4608 (before the /2 divider segment.)

 

However at the output I am getting 2292.43MHz, but I am not exactly sure how that value is being calculated.

Are my calculations correct?

 

Thank you,

-Oscar

  • 0
    TI__Guru** 108480 points
    Hi Oscar,

    What is is your reference clock frequency? If it is 122.88MHz, did you check that it read 122.88MHz in your test equipment?
  • 0 in reply to Noel Fung
    TI__Intellectual 2245 points

    Hi Noel,

    My reference is 122.88 MHz and yes I made sure to check that the 122.88MHz is present.

    Regards,

    Oscar

  • 0 in reply to Oscar Guevara
    TI__Guru** 108480 points
    Hi Oscar,

    Your PLL frequency planning is correct, it should work. If your test equipment also read 122.88MHz for the reference clock, then you should get exactly 2304MHz from the synthesizer. Another thing you can try is put PLL_NUM=0, i.e. an integer channel, see if you can get what is expected. If you cannot get what you want, please provide your register setting for us to review.
  • 0 in reply to Noel Fung
    TI__Intellectual 2245 points

    Hi Noel,

    I tried the setting PLL_NUM to 0 and I do get what I expect. However the issue persists when I try to add in the fractional portion. .Attached is the configuration that I am using in order to try to obtain 2304 MHz from the LMX.

    LMX_2304.cfg

    Thank you,

    Oscar

  • 0 in reply to Oscar Guevara
    TI__Guru** 108480 points

    Hi Oscar,

    I think you have missed programming some registers, for example, R2 and R4. Furthermore, there are some register bits not correctly set.

    Please try the attached setting. I assumed you will use RFoutA in the setting.

    Fullscreen e2e.txt Download 
    R64	0x4000AF
R62	0x3E0000
R61	0x3D0001
R59	0x3B0000
R48	0x3003FC
R47	0x2F00CF
R46	0x2E0FA3
R45	0x2D0003
R44	0x2C0000
R43	0x2B0000
R42	0x2A0000
R41	0x290004
R40	0x280000
R39	0x278204
R38	0x260024
R37	0x254000
R36	0x240411
R35	0x23021B
R34	0x22C3EA
R33	0x212A0A
R32	0x20210A
R31	0x1F0401
R30	0x1E0034
R29	0x1D0084
R28	0x1C2924
R25	0x190000
R24	0x180509
R23	0x178842
R22	0x162300
R20	0x14012C
R19	0x130965
R14	0x0E018C
R13	0x0D4000
R12	0x0C7001
R11	0x0B0018
R10	0x0A10D8
R9	0x090302
R8	0x081084
R7	0x0728B2
R4	0x041943
R2	0x020500
R1	0x010808
R0	0x00229C