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LMX2581: PLL locking failure issue

Taehee Kim0
Prodigy 20 points
Part Number: LMX2581

Dear,

i'm using the LMX2581 product and emulating in my application board.

i'm trying PLL 150MHz locking with reference clock 20MHz and using the internal VCOfor.

reference clock 20MHz

output : 150MHz

but locking was failed.

Could you check the setting value. my resister setting is below.

R5 (INIT) 0x40870010
R15      0x021FE7BF
R13      0xF082C10D
R10      0x210050CA
R9        0x03C7C039
R8        0x207DDBF8
R7        0x1C082307
R6        0x000004C6
R5        0x00012805
R4        0x00060004
R3        0x2019E543
R2        0x04000002
R1        0xC00020A1
R0        0x441A0000

  • 0
    TI__Mastermind 28001 points

    Tahee,

    TicsPro is a useful tool for debugging these register related questions.  You can type these values in the "registers" tab or import them.

    I did so and I found that based on register R5 and R1, you have VCO_SEL_MODE to force the VCO, but VCO_SEL is set to "VCO1".  But it looks like your VCO frequency is 2625 MHz, which VCO1 can not hit.  Also, you were using the 2nd order modulator with a fraction of "0/0", which might cause some issues.

    For the best phase noise, I would use TICSPro with a 1st order modulator and a phase detector frequency of 25 MHz  The fractional spurs should be 5 MHz away and there will be no added nose from teh delta sigma modulator.

    Regards,

    Dean