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Original question:

LMX2572: RAMP GENARATION

LMX2572: Sub-Fractional Spurs

Koshi Ninomiya
Expert 4350 points
Part Number: LMX2572

Hi Team,

My customer is evaluating LMX2572 on their ES board, and they have a question about sub-fractional spurs.
When they evaluated the 2nd  ES board, sub-fractional spurs of LMX2572 became around 5dB higher than that of the 1st ES board.
They do not change application of LMX2572 and settings, they also use MASH_SEED with the same setting.
It is a general question,
Q1: Does the sub-fractional spurs vary by variation of LMX2572 characteristic, part to part, or lot to lot?

Thank you.

Best Regards,

Koshi Ninomiya

  • 0
    TI__Guru* 88520 points

    Hi Ninomiya-san,

    Do you have their configuration such as phase detector frequency, VCO frequency, etc?

  • 0 in reply to Noel Fung
    TI__Expert 4350 points

    Hi Fung-san,

    Their configuration is as follows,

    Option-1: 5523MHz_190315.txt

    5523MHz_190315.txt 
    R78	0x4E0001
R75	0x4B0800
R71	0x470081
R57	0x390020
R46	0x2E07F1
R45	0x2DCE1E
R44	0x2C3F23
R43	0x2B0017
R42	0x2A0000
R41	0x290051
R40	0x280000
R39	0x270064
R38	0x260000
R37	0x258305
R36	0x240037
R34	0x220010
R30	0x1E18A6
R29	0x1D0000
R14	0x0E1878
R12	0x0C5001
R11	0x0BB018
R10	0x0A10F8
R9	0x091004
R0	0x00231C

    Option-2:5629r25MHz_190315.txt

    5629r25MHz_190315.txt 
    R78	0x4E0001
R75	0x4B0800
R71	0x470081
R57	0x390020
R46	0x2E07F1
R45	0x2DCE1E
R44	0x2C3F23
R43	0x2B0075
R42	0x2A0000
R41	0x290111
R40	0x280000
R39	0x270190
R38	0x260000
R37	0x258305
R36	0x240038
R34	0x220010
R30	0x1E18A6
R29	0x1D0000
R14	0x0E1878
R12	0x0C5001
R11	0x0BB018
R10	0x0A10F8
R9	0x091004
R0	0x00231C

    Thank you.

    Best Regards,

    Koshi Ninomiya

  • 0 in reply to Koshi Ninomiya
    TI__Guru* 88520 points

    Hi Ninomiya-san,

    in Option 1, are you referring to 1MHz and 500kHz spurs?

    In Option 2, 250kHz and 125kHz spurs?

    In both cases, you can set the fractional DEN to an irrational number such as 400001234 to eliminate these spurs.